This has to do with which devices are authorized, when you install a copy for beta iTunes may not recognize it as being "owned" by that account. 

In this case you can follow these steps to correct the problem:

1)  Remove ALL copies of Swipe from your iPhone/iTouch AND from within iTunes

2)  From the "Store" menu select "Deauthorize computer"

3)  Install the Beta copy of Swipe and it's associated mobile provisions file all sent to you by AppNinjas.

4)  Again from the "Store" menu select "Authorize computer"

5)  Reconnect your iPhone/iTouch (if it's not already) and sync.  At this point the application should install and be ready to go.

You MIGHT have to repeat the above steps a second time to make it properly re-authorize ALL the necessary files.