If you want to go to a different campaign in the single player mode without finishing the one before (i.e. If you want to do the Borg campaign without finishing the Federation one) you can. Simply put your mouse over the campaign you wish to do, and click on it. A menu will come up showing the first mission in the campaign. Click on the mission, then click "OK". You can then do that mission. Once the mission is completed, the game will automatically take you to the next mission.