How do I add to a previous support post?

I submitted a question earlier this morning and Jack Kitterhing responded. I want to take him up on the offer to help me with developing a FB app so I can use Facebook Ultimate.

Here is the link to the original post.

Here are my follow-up issues:

I'm following the video tutorial for creating an app. However, when I enter the App Domain of "" I get this error message:
"You have specified an App Domain but have not specified a valid integration URL. must be derived from one of : Site URL, Mobile Site URL, Canvas URL, ... etc."
Where do I go from here?