If you are not running version 0.5.2 - which was released last night, then go to planeshift.it, look in the upper left corner and click on the dowload now icon.
You need to pick the right file for your os ( windows, mac, linux)
If you are running 0.5.2 and are seeing a brown window that says that you need to install an update the run the pslaunch application. It should be in the same directory as psclient