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