Start a LAN game and lower the console by pressing the key below ESC or the key you have bound to console. Type editor.open(); in the console or try ALT + E. Mapping tutorials can be found at the URLs that follow:

http://www.planettribes.com/necrobones/tribes2/tutorial.html
http://web.lemuria.org/T2beta/Mapping/  
http://sierrastudios.com/games/tribes2/memanual.html
http://hosted.tribalwar.com/zear/mapeditmain.html