Document Data sources are mapped to forms in the current application.  If there are no forms defined in the current application then there is nothing to choose from.

If this was a new application then this would be the correct experience.  You will need to create a Form and View and THEN create your first XPage's 

Follow this free tutorial and it will walk you through the complete process for a simple application. 

http://www.tlcc.com/admin/tlccsite.nsf/coursedetails.xsp?ccode=XP85INTR 

 If there are existing forms in that application then they should appear in the "form" field dropdown when creating a document data source yes.

If they don't, try restarting notes/designer.  Might just be a hiccup. 

 I don't know of anything that would supress the display of forms in the list.

Possibly run fixup on the application as maybe for the form design view is corrupt. 

 

I can highly recommend the courses from TLCC I have taken a couple (I receive no remuneration for this). Also I would strongly recommend two books "Mastering XPages" and "XPages Extension Library".

The XPages hill is pretty steep, but climbable.  Basic documentation is pretty sparse, but between the courses, the books and a lot of internet searching I'm getting there.