I'm guessing you need to install the free version (Make).
I think they have made it that if you install Make on Windows you get a free trial of pro then it reverts to Make, but if you install pro, it's free for the trial period but then stops working. From what I know Mac reverts.
Removing Pro and installing Make should have no affect on your extensions as they are stored in a separate folder.
You can download Make from here.
http://www.sketchup.com/download/all