From the guide.
4. The next step (once the update is finished) is to set the permission bits so that you can run the client. You'll need to set the permission bits through Terminal. Open Terminal, located at /Applications/Utilities/Terminal(.app). In the Terminal window, type "cd " (note the space " " after "cd"). Then, locate your Planeshift 3D folder in the Finder and drag it into the open Terminal window. Press return.
Then type:
"chmod 755 psclient.app/Contents/MacOS/psclient"
and press return.
And then type:
"chmod 755 pssetup.app/Contents/MacOS/pssetup"
and press return.
Finally, type
"chmod 755 updater.app/Contents/MacOS/updater"
and press return.
That is the most common problem.