The server has to have the map you want to play. If you just want to play it by yourself or LAN, and its NOT in your map list its because the maps dont have a .arena file. None of my maps have it for example, so you will have to turn the pure server option off and type /map [mapname] in console. Bear in mind the pk3 and the map name might be different. For example to play waveformb, it would be /map waveformb. Even though the pk3 is called waveformb1