Honestly, it is impossible to tell what the cause of the problem is.
Where I cannot reproduce the problem on any of my computers running Windows 7 in both 32 and 64 bit modes, it would suggest something unique to your system.
Perhaps there is popup blocker software, or anti virus software that is preventing the data assist wizard from running normally.
If you have any sort of real time file scanning software, try disabling it before running the DataAssist wizard.
For your other issue,
If you delete columns from your table after running the data assist wizard, you will need to remove all references to those columns from your pages.
on the results page, remove the code that displays those columns:
<?php echo $row_RecordsetName['ColumnName']; ?>
on the insert and update page, remove the form elements for those columns,, you will also need to edit the insert and update server behaviors to remove the references to those columns.