okay, let's say your form looks something like this...
<form name="frmTest">
<input type="text" name="frmText1">
</form>
then in your popup you are going to be able to read that value by using this:
var someValue = window.opener.document.frmTest.frmText1.value
That's breaking it down "Barney Style". Note the opener portion... this is the key element to accomplishing what you are trying to do. Let me know if need more help.