Open "campaignstate.lua" in the " \dawn of war\profiles\(your profile)\w40k\singleplayer\ campaign\state1" folder then find the lines "maxmission =" and "scenario =". Change the values of these two lines to the number that corresponds to the level you want to play.
Cheat codes:
Press Ctrl + Shift + ~ while playing and enter the following.
| cheat_power( n ) | Get n amount of power |
| cheat_requisition( n ) | Get n amount of requisition points |
| cheat_killself | Suicide |
| cheat_revealall | Turn fog of war off |