It's in the "descr_character.txt" file in the data folder (can't remember if you have to unpack it or not...).

In MTW, you change the very first "global" setting for default movement points. Any change impacts everything. Individual character settings have no effect.

In M2TW though you have to change the "starting action points" for EACH type of character-- just the opposite of MTW. I think this is better as you can individually change the movement of agents, admirals (fleets), etc. For example, I doubled "just" my ship movement but left everything else the same.

Hope this helps.