Had same problem applying patch. Found that the update was looking for the Parallels application in the Applications folder. I had previously relocated the Parallels application into an Applications subfolder named "Parallels" which included the other Parallels utilities (Image Tool, etc). Evidently, the update doesn't like that.
Moved Parallels app back into the Applications folder and the update completed successfully.
Hope this helps.
Having the same problem under OSX 10.5.6. My App is correctly situated in the APP folder but I'm still having the same problem, none of the previous fixes suggested have worked for me.