Hi

How do I make an item collection tag (i saw it in a previous discussion but can't find it anymore)

Also what is the command for an ai to run over and enter a vehicle seat? Am currrently using magic load which isn't very good if it happensn right in front of you.

for item_collection tag, open guerrila, hit ctrl-n, select item_collection, and then when the untitled tag appears, add an item permutation. The rest should be self-explanatory (other than weight, which is the chance in percentage that an object will spawn if you have multiple item permutations for the item_collection tag).
As for the command to make an ai run over and get in a vehicle, use this:
(ai_go_to_vehicle the_jacks warthog_mp_1 "W-driver")
where the_jacks is the encounter, warthog_mp_1 is the vehicle and W-driver is the seat which you want them to go to (copyright halo script bible).

I use:
(unit_enter_vehicle BIPED VEHICLE "SOMETHING-driver")

^^
Assuming that the biped of the AI is named, you can also consider this.

If you just need a one or a few guys to enter a vehicle or vehicles, creating named bipeds and attatching them to encounters is the way to go. Then just do the script above, and the biped(s) will enter the vehicle(s) and start doing their stuff.
Alternatively, use a command list to have them enter the vehicle. I do this with my shade gunner grunts in firefight-descent (wip).