member/skins/스킨/signup_form.html 에서

<div class="control-group">
 <div class="control-label">{$lang->allow_mailing}</div>
 <div class="controls" style="padding-top:5px">
  <label for="mailingYes"><input type="radio" name="allow_mailing" id="mailingYes" value="Y" checked="checked"|cond="$member_info->allow_mailing == 'Y'" /> {$lang->cmd_yes}</label>
  <label for="mailingNo"><input type="radio" name="allow_mailing" id="mailingNo" value="N" checked="checked"|cond="$member_info->allow_mailing != 'Y'" /> {$lang->cmd_no}</label>
 </div>
</div>

라는 형태로 되어 있는 부분을 

<div class="control-group">
 <div class="control-label">{$lang->allow_mailing}</div>
 <div class="controls" style="padding-top:5px">
  <label for="mailingYes"><input type="radio" name="allow_mailing" id="mailingYes" value="Y" checked="checked"|cond="$member_info->allow_mailing != 'N'" /> {$lang->cmd_yes}</label>
  <label for="mailingNo"><input type="radio" name="allow_mailing" id="mailingNo" value="N" checked="checked"|cond="$member_info->allow_mailing == 'N'" /> {$lang->cmd_no}</label>
 </div>
</div>

형태로 바꾸면 되요

$member_info->allow_mailing  값 부분을 반대로 바꾸면 될겁니다.