You can try cloning the following html that you have on your main login page into the modal box:
<form method="post" action="/">
<div style="margin-top:-4px;">
<a href="javascript:void(0);" onclick="$( #loginFormMiniMain ).fadeOut( fast );">X</a>
<input type="hidden" name="RET" value="-1">
</div>
<table cellpadding="0" cellspacing="6" border="0" align="center">
<tbody><tr>
<td align="right">Username</td>
<td><input type="text" class="input" id="username" name="username" size="15" value="">
<input type="hidden" name="auto_login" value="1"></td>
</tr>
<tr>
<td align="right">Password</td>
<td><input type="password" class="input" name="password" size="15"></td>
</tr>
<tr>
<td colspan="2" align="right">
<a href="http://expressionengine.com/forums/member/forgot_password/">Forgot your password?</a>
<input type="submit" class="submit" value="Login">
</td>
</tr>
</tbody></table>
</form>