I was having the same problem; here is what the Sketchup support team sent to me; issue is fixed now:

Uninstall Google SketchUp Pro 8 by visiting our Uninistall instructions. I'm suspicious that the plist preferences file has something to do with the issue as well.
After uninstalling, we suggest repairing disk permissions:
Restart your computer.
Open a Finder window.
Click Applications.
Click Utilities.
Double-click Disk Utility.
Select your hard drive.
Select the First Aid tab.
Click Repair Disk Permissions.
After the permissions are repaired, exit Disk Utility.
Please repeat steps "a" through "i" once more.
Log in to your computer with a user account that has administrative privileges.
Please visit our Google SketchUp Pro page to download the Google SketchUp Pro 8 install file.
When the download completes, double-click on the install file and follow the on-screen instructions.