It looks like you've been waiting for an answer for a bit. I'm not able to tell what the issue is from the screenshot. Are you able to reproduce the issue using any sample data? Or can you mock up a similar blend using two generic data sources?
I had the same problem. It seems that if the linking symbol overlaps the name of the field you cannot just use a single click. My solution was just to scroll to the right of the dimensions window using the bar on the bottom. Then the linking symbol no longer overlaps the field name, and you can single click as you desire.
I hope this works for you.