Hello,
I am on OSX 10.8.4.
Sketchup was installed as admin. There are only default plugins in the plugins folder, no old sketchup.rb file. I only use Sketchup as Admin. All works fine on the older MacBook with same OS version.
There is no Ruby Console menu item or any other menu item showing between Hide Dialogs and Bring All to Front in the Windows menu so I can't try the Ruby command.
It does seem like a permissions issue. I have another piece of 3rd party software that crashes on launch on the new computer and gave a permissions related error when trying to activate it. I have repaired permissions a few times with Disk Utility and Techtool Pro. Full read/write for me and admin from Applications folder down to Tools folder where sketchup.rb file is.