You need to go into the insert server behaviour.
Open up server behaviours panel and double click on "insert"
Leave the first screen as is and click next.
You should see the newly added dropdown on the lft. Highlight it and then click the lightning bolt underneath on the right. Scroll down to the form and open it, then click on the dropdown field. Click ok all through and save the page. It should now work.
I'm off out now so perhaps someone else can help you further if you still have difficulties.