Some themes by default don't properly show radio buttons. For example your options might looks like this and be unclickable:
This is actually a fairly easy fix :-) All you need to do is insert the follow bit of CSS in the Custom CSS box on the Display settings pate.
Step 1 - Go to the Display Settings page
Step 2 - Scroll down to the Custom CSS area
Step 3 - Insert the following bit of code