It should automatically split when you have that version of the game and livesplit is set to Halo: Combat Evolved with the autosplitter activated. Could you take a screenshot of the menu that pops up when you right click livesplit and click edit splits?
Here's a link for it... i think all the settings are correct, but it won't automatically start the timer when I start the campaign. Do I have to manually click it to start? I feel like an idiot cause I can't figure this out.