At the purchase menu, click on the scroll bar and scroll down until you see the desired mech, weapon, or vehicle. While you are holding the Left Mouse Button, move the pointer on top of the desired item. While still holding the Left Mouse Button, right click on the desired item. You should be able to buy one of that item. After buying one, you should be able to buy an unlimited supply of it. When used with the rockandrollpeople and poundofflesh codes, most missions are much easier.