membership download protection

can *anyone* please tell me how download protection is supposed to work in the membership plug-in? There is as usual ABSOLUTELY NO DOCUMENTATION about this feature ANYWHERE that I can find.

I have (several) 100MB files in a directory called ....../wp-content/blahblah/

I have set (in the membership options page) the actual download URL:

http://channelrobot.com/wp-content/blahblah

and the masked download URL to:

http://channelrobot.com/obsfucated

I've tried setting the URL in my post to:

http://channelrobot.com/wp-content/blahblah/bigfatfile.rar

no success its just plainly visible....

Ive set it to

http://channelrobot.com/obsfucated/bigfatfile.rar

...now WP cant find the file

I've created a "download group" - whatever that means, in my membership level and I've tried to use this like a short code around my download URL, - just looks silly and does nothing, I've replaced my URL with the "group name" - give me an invalid URL and thus no access to the file...

Now I'm out of ideas...

Pls someone tell me what I'm doing wrong here,put me out of my misery with this plug-in, pls pls pls someone point me at some usable documentation!!!!.