you are supposed to hit the select button. But you need to be right on top of the other player. Also if you don't hear a beep sound when you hit select its not working.

Get close to your teammate and press the corresponding button (Select for PS3 and A for 360) to request either ammo or medical supplies (Depends on the teamamte's class).

All the above answers r true but it also depends on the person with the ammo or med kit because when they get the request they may chose to ignore you. You may ask them for ammo or meds a hundred times but it they don't want to drop the kit, they won't. It sucks that some people aren't team players but that's the way some people are. One way to kind of make sure u get ammo or meds is to get a headset or earpiece and tell them give you the kit or play with friends that will help u out when you need it.