It will let you know you got one, then you go into your caps lock menu, under where you pick your kit, you will see a flashing tab for field upgrade above your map, from there you pick the upgrade you want, then add it to your personell inventory, which will be available once you choose the respective kit. It lasts until you leave the server.
But there are some rocks and windows of buildings that you can blow up and get field upgrades for weapons. I found the Laser Einfeld the other day. Muh ha ha ha ha.
They are upgrades you get for working as a squad... aka doing what the squad leader actually tells you to do. If you do it enough, you get an unlock that you can use only while you are connected to that server. Great way to test out other weapons.