Ok... here's your problem.
This: <input name="hiddenField" type="hidden" id="hiddenField" value="MM_insert" />
Needs to be changed to this:
<input name="MM_insert" type="hidden" id="MM_insert" value="MM_insert" />
Because the flag is looking for is an input with the name of "MM_insert". It looks like you put it in the value rather than the name... the script though is looking for an input with that name. It's not concerned what the value is - just that variable exists.