Attach button not working

I was having this same issue. The fix, for me at least was to create a new form, create any necessary fields and then go to another WP Admin page (any one) and then return back into the Custom Contact Forms admin page. Then the button worked. It would appear it refreshes some aspect of the code.

I found the same problem, but here is how I got around it;

1. Sometimes I had to refresh the page after adding new fields/field items. This could be as simple as just clicking on the link in the left bar to go back to the forms page.
2. Most of buttons need a long mouse press before they actually work. Wait for the button to “depress” before letting go of the mouse button.