The files you're looking for are data\levels\mission06\xml\world.xml and data\levels\mission06\mission.xml.
Though I'm not quite sure how, but try this:
loyalists3 -> us_marines_squad3
and in mission.xml
support_loyalists -> support_us_marines
This should change the player-controlled squad into marines instead of loyalists.
You can actually add 4 additional 3-people squads (controllable, either marines or loyalists or both) just by editing these two files.
Ok, i edit the world.xml and mission.xml and change the loyalist for the marines. But the mission crash. This is the error:
Wed Oct 31 23:11:48 2007
Crash in application version: 30621.2989
data\lib\units\extensions\groupunit.dsf(-1): cant find member: _side in type <void>
Physics : threaded
Any idea what is wrong? Thanks for help me....