We are currently working on a temporary issue where users of SketchUp 2013 and 2014 can’t open the software and are forced to download SketchUp Pro 2015. There is a temporary fix:
Shut down SketchUp entirely
Disconnect from the Internet (WiFi and hardwired)
Restart SketchUp
You should then be able to start SketchUping.
We hope to have this resolved soon.