1. Do you have the full version of the game? If you don't, you won't have the maps for a multiplayer game. Most servers only use the default maps, so if you're having this issue everywhere, you probably have the demo.
2. If you do have the full game, then you might be playing on a server that is using custom maps. There are a number of locations to get the maps. First check to see if there is a website given in the details for the server. That is the best first choice, as it will generally have a pack for all the mods on that server. You can also check a couple other sites, including mine: treadmarks.vorklift.com - lot of custom maps available.