Use the crack found here: http://darkblizz.org/Forum2/sc2-tools/cracked-sc2-exe-and-mapeditor
to gain access to the map editor, download the updated Blizzard maps from here: http://www.mediafire.com/?itcmmnjyijz
(in the outdated maps the xel'naga towers/gold minerals are missing), open a map, select 'Map' -> 'Player properties' and set one of the players (not player 1 because its you) 'Control' to be a computer and 'AI' to be a computer too (there you also select name, color, race, and start location for you and for the AI, leave Decals to stay 'Default'). Make sure Player 1 'Control' is set to 'User' (no need to set anything for Player 1 in 'AI').
Next, select 'Modules' -> 'Triggers' than right click on 'Events' (orange flag) -> 'New' -> 'New Action' than on the left select 'Player' and on the right select 'Set Player Difficulty Level' and press OK.
in 'Set player _' be sure to select the number of the player you set to be a computer and in 'difficulty level to _' select the difficulty level you want to play (easy, medium, hard etc.)
DONE! all you have to do now is to select 'File' -> 'Test Document' and you are in the game! (don't panic if the "validate SC2 license..." pops up, it should disappear after a couple of seconds).