Look for the Forms App in your Ning Network's Ning App Directory (coming soon), via Dashboard » Features » Add Ning App.

Alternatively, you can manually install the Forms App, by going to:
http://www.yourNingNetworkDomain.com/opensocial/ningapps/addByUrl

Then enter the following app URL:   http://apps.io/forms/

Or to use a custom app title, use:   http://apps.io/forms/?title=Custom+Title