There is a bug in myBalsamiq where. if you are using any of the new controls (Line of Text or Block of Text) in your Mockups, the editor doesn’t know what to do with them, and then won’t let you do anything at all.
The fix we have right now is to swap those controls for Text Controls in Mockups 3, re-export them, and then re-upload them. We know that fix really sucks, so we are working to add those controls to myBalsamiq. If you would like, you can send us the BMPR file, and we will swap the controls, and then re-export the file for you. Whatever is easiest for you.
I’m really sorry about the snag with this. We are going to get it fixed ASAP.