When you add doXtop as an application to a Ning site - Ning doesn't know - which user you are on doXtop the first time. So Ning cannot connect you automatically - and shows therefor an empty box - like this one:
To connect please press the green Publish button above - then you get the following screen ...
If you are already a doXtop user - just type your doXtop user Name & password in the upper area - and Ning will connect you to doXtop. if you are not yet a doXtop member - you can create a doXtop account by typing in your data in the lower part of the form - and press the Create button.
From then on doXtop will always show all the connected users publications automatically - if you sign in into your Ning site. Note: If you didn't publish any documents yet - there's nothing to show as well.