IIRC, the permissions for the applications folder are such that only root can modify it. (There needs to bea little "unlock" button on it or something in final.)
Anyhow, my details might not be exactly right, but that's close. You need to change the folder permissions to allow your user to modify it. (Or open a copy of desktop.app as root with Pseudo...)
Search the fora to find out how to do these things.