right.... see if this makes sense -
depending on what button you press you are creating a variable name and a value for that variable that gets taken to the next page.
using your buttons you are creating a variable called either '$NUKE' or '$PNUKE' or '$PHPBB' , except that in this case whichever button you are pressing the variable is getting the name '' (as in nothing) because $NUKE, $PNUKE and $PHPBB are not set in your index.php.
anyway, try the following code .....
replace -
<input name="$NUKE\" type=\"submit\" value=\"PHP Nuke\">
</td>
<input name=\"$PNUKE\" type=\"submit\" value=\"Post Nuke\">
</td>
<input name=\"$PHPBB\" type=\"submit\" value=\"PHPbb\">
with -
<input name=\"nuketype\" type=\"submit\" value=\"PHPNuke\">
</td>
<input name=\"nuketype\" type=\"submit\" value=\"PostNuke\">
</td>
<input name=\"nuketype\" type=\"submit\" value=\"PHPbb\">
and in your main.php file replace
"<input id='type' type='text' name='type' readonly='true' size='30' maxlength='100' value='$HTTP_POST_VARS'>"
with
"<input id='type' type='text' name='type' readonly='true' size='30' maxlength='100' value='$nuketype'>"
then you should get the result you desire.