Well... How about just redownloading the whole client? That is usually the easiest way if you have problems updating.
Or if you want to try, you can open the Terminal app from the Utilities subfolder of your Applications folder. Next, type "cd /Applications/planeshift" (without the quotes, assuming you have ps in /Applications/planeshift) after that (you will need to be on an admin account at this point) type "sudo open pslauncher.app" (again, without quotes), enter your password and you should have an open pslauncher with admin rights. However, if you have problems updating, try "sudo open pslaunch.app --args --repair". If you do it that way, it won't open any windows, the pslaunch icon will just appear in the dock. If you want to check what it's doing, take a look at the Console application in /Applications/Utilities.
Please ask for clarification if you have trouble understanding that, my explainings tend to confuse the "victim" more than it already is.