Is it possible to download the scenes that others have designed - I know there was a group in the proper download section - but with an editor available I am sure people have designed their own and I think I am missing out on some of the action - can anyone put me right?

There's a whole topic dedicated to custom scenarios:

http://massiveassault.com/forum/viewforum.php?f=14

There aren't too many to chose from, however, as the primary focus of people on this forum have been multiplayer, and as of yet you can't create multiplayer scenarios.