제가 수정한 소스로 한번 해 보세요..
일단 다른데서 가져오신거라 form 이름이 안 맞았고요.
괄호가 안닫힌 곳도 있었고...암튼 몇군데 좀 바꿨습니다 ㅋ

<form name='frm'>
<select name="mb_5" size="1" onChange="redirect(this.options.selectedIndex)">
    <option value="00">선택</option>
    <option <? if ($member[mb_5]=="10") { echo "selected"; } ?> value="10">정책</option>
    <option <? if ($member[mb_5]=="11") { echo "selected"; } ?> value="11">산업/기업</option>
    <option <? if ($member[mb_5]=="12") { echo "selected"; } ?> value="12">전문가</option>
    <option <? if ($member[mb_5]=="13") { echo "selected"; } ?> value="13">동서남북</option>
    <option <? if ($member[mb_5]=="14") { echo "selected"; } ?> value="14">예규/판례</option>
    <option <? if ($member[mb_5]=="15") { echo "selected"; } ?> value="15">조세</option>
 </select>
 <select name="mb_6" size="1">
    <option value="">내용없음</option>
<?
    if ($member[mb_6]=="10100") { $select2_name="재정"; } 
    elseif ($member[mb_6]=="10101") { $select2_name="예산/감사"; }
    elseif ($member[mb_6]=="10102") { $select2_name="금융/증권"; } 
    elseif ($member[mb_6]=="10103") { $select2_name="공정거래";    } 
    elseif ($member[mb_6]=="10104") { $select2_name="기타"; } 
    elseif ($member[mb_6]=="11100") { $select2_name="기업일반"; } 
    elseif ($member[mb_6]=="11101") { $select2_name="산업정책"; } 
    elseif ($member[mb_6]=="11102") { $select2_name="IT"; } 
    elseif ($member[mb_6]=="11103") { $select2_name="벤처중기"; } 
    elseif ($member[mb_6]=="11104") { $select2_name="과학/바이오"; } 
    elseif ($member[mb_6]=="11105") { $select2_name="유통/소비자"; } 
    elseif ($member[mb_6]=="11106") { $select2_name="주류"; } 
    elseif ($member[mb_6]=="11107") { $select2_name="기타"; } 
    elseif ($member[mb_6]=="12100") { $select2_name="세무사"; } 
    elseif ($member[mb_6]=="12101") { $select2_name="회계사"; } 
    elseif ($member[mb_6]=="12102") { $select2_name="관세사"; } 
    elseif ($member[mb_6]=="12103") { $select2_name="기타"; } 
    elseif ($member[mb_6]=="13100") { $select2_name="재경부"; } 
    elseif ($member[mb_6]=="13101") { $select2_name="행자부"; } 
    elseif ($member[mb_6]=="13102") { $select2_name="금감위"; } 
    elseif ($member[mb_6]=="13103") { $select2_name="공정위"; } 
    elseif ($member[mb_6]=="13104") { $select2_name="국세청"; } 
    elseif ($member[mb_6]=="13105") { $select2_name="관세청"; } 
    elseif ($member[mb_6]=="13106") { $select2_name="기타";    } 
    elseif ($member[mb_6]=="14100") { $select2_name="예규"; } 
    elseif ($member[mb_6]=="14101") { $select2_name="판례";    } 
    elseif ($member[mb_6]=="14102") { $select2_name="심판례"; } 
    elseif ($member[mb_6]=="14103") { $select2_name="감사원심사례";    } 
    elseif ($member[mb_6]=="14104") { $select2_name="상담사례"; } 
    elseif ($member[mb_6]=="15100") { $select2_name="내국세"; } 
    elseif ($member[mb_6]=="15101") { $select2_name="지방세"; } 
    elseif ($member[mb_6]=="15102") { $select2_name="관세"; } 
    elseif ($member[mb_6]=="15103") { $select2_name="기금/부담금"; } 
    elseif ($member[mb_6]=="15104") { $select2_name="사회보험"; } 
    elseif ($member[mb_6]=="15106") { $select2_name="해외조세"; } 
    elseif ($member[mb_6]=="15107") { $select2_name="기타"; } 
    else { $select2_name="선택안함"; }

echo "<option selected value=\"$member[mb_6]\">$select2_name</option>";
?>
</form>

<script language="JavaScript">
<!--
var groups=document.frm.mb_5.options.length;
var group=new Array(groups)

for (i=0; i<groups; i++)
    group[i]=new Array()
    group[0][0]=new Option("내용없음","")
    group[1][0]=new Option("내용없음","")
    group[1][1]=new Option("재정","10100")
    group[1][2]=new Option("예산/감사","10101")
    group[1][3]=new Option("금융/증권","10102")
    group[1][4]=new Option("공정거래","10103")
    group[1][5]=new Option("기타","10104")
    group[2][0]=new Option("내용없음","")
    group[2][1]=new Option("기업일반","11100")
    group[2][2]=new Option("산업정책","11101")
    group[2][3]=new Option("IT","11102")
    group[2][4]=new Option("벤처중기","11103")
    group[2][5]=new Option("과학/바이오","11104")
    group[2][6]=new Option("유통/소비자","11105")
    group[2][7]=new Option("주류","11106")
    group[2][8]=new Option("기타","11107")
    group[3][0]=new Option("내용없음","")
    group[3][1]=new Option("세무사","12100")
    group[3][2]=new Option("회계사","12101")
    group[3][3]=new Option("관세사","12102")
    group[3][4]=new Option("기타","12103")
    group[4][0]=new Option("내용없음","")
    group[4][1]=new Option("재경부","13100")
    group[4][2]=new Option("행자부","13101")
    group[4][3]=new Option("금감위","13102")
    group[4][4]=new Option("공정위","13103")
    group[4][5]=new Option("국세청","13104")
    group[4][6]=new Option("관세청","13105")
    group[4][7]=new Option("기타","13106")
    group[5][0]=new Option("내용없음","")
    group[5][1]=new Option("예규","14100")
    group[5][2]=new Option("판례","14101")
    group[5][3]=new Option("심판례","14102")
    group[5][4]=new Option("감사원심사례","14103")
    group[5][5]=new Option("상담사례","14104")
    group[6][0]=new Option("내용없음","")
    group[6][1]=new Option("내국세","15100")
    group[6][2]=new Option("지방세","15101")
    group[6][3]=new Option("관세","15102")
    group[6][4]=new Option("기금/부담금","15103")
    group[6][5]=new Option("사회보험","15104")
    group[6][6]=new Option("해외조세","15106")
    group[6][7]=new Option("기타","15107")

var temp=document.frm.mb_6;

function redirect(x){
for (m=temp.options.length-1; m>0; m--){
temp.options[m]=null
for (i=0;i<group[x].length;i++){
  temp.options[i]=new Option(group[x][i].text,group[x][i].value)
}
temp.options[0].selected=true
    }
}
 //-->
</script>
</td>