Hey !
Try this:
Rename the current workspace you are using, so when CE starts, it cannot
find it and gives you the switch workspace dialog. create a new one. if
possible.
If it's not a workspace issue, close CE, go into the CityEngine app
(right-click on app, Show Contents), then search for the '.cityengine'
metadata folder and delete it. you may need to make invisible files visible.
e.g.
http://ianlunn.co.uk/articles/quickly-showhide-hidden-files-mac-os-x-mavericks/
Matthias Buehler
Head of 3D Technologies
twitter: @MattB3D