When I attempt to add an XMP file to a Project Navigator project:
In order to add an XMP source to a Project Navigator project, you need to have write permissions to the XPS source directory and Temp Directory. You do not need to have administrator privileges to add XPS source.
You need to have write permission to the directory from which you are adding the XPS source, as EDK generates a TCL script in that directory to change Device settings. If you do not have write permission, this process fails.
Also, you need to have write permissions to the system temporary directory (most often picked up from environment variable TMPDIR or TEMP), as EDK tries to write some files to this temporary directory. If you do not have write permission, the process to add the XPS source fails.
NOTE: Once you add the XPS source under admin mode and you try to do the same under non-admin user mode, it fails because the files EDK has already written to are not writable by a non-admin user, and the process fails.