Thought this might benefit someone else new to WPMU plugins, so here it is.....

If you install Supporter and you navigate to "Premium Extensions" and then get an unfriendly message:

cheatin' uh?

Its simply because you have WordPress set-up to not display the plugins directory, I think its WordPress attempt at poor humour with regards to its lack of descriptive text because it certainly offers no help to an admin or end user.

Navigate to:

Network Admin ---> Settings ---> Settings

Under Menu Settings ensure that "Enable administration menus" is checked then save and the message should disappear!

Q to devs, could the plugin not perform a simple check and enable the option if it is disabled? Could be off putting if unnoticed and users stumble across it.

Hope it might help someone. :-)