Custom ploicy added via a *.adm file is not visible

Hey all,
I am trying to add a custom Policy to a GPO so that I can set the users background colour.
I have the below code, and I have successfully imported the *.adm file without error, but the policy is not being shown.
If I change '!!categoryname' to something unique, a folder is created in the GOP, but still no policy.
Can someone please help me find what is wrong with the script.
Thanks.
CLASS USER

CATEGORY !!categoryname
KEYNAME "Control Panel\Colors"
    POLICY !!policyname
        EXPLAIN !!explaintext
        PART !!labeltext DROPDOWNLIST REQUIRED
            VALUENAME "Background"
            ITEMLIST
                NAME "Black" VALUE "0 0 0"
                NAME "Blue" VALUE "58 110 165"
            END ITEMLIST
        END PART
    END POLICY
END CATEGORY

[strings]
categoryname="Desktop"
policyname="Set the desktop background colour"
explaintext="This policy sets the background colour of the desktop."
labeltext="Choose a colour"