How can I use underslung attachments in Battlefield 3?

I have unlocked the Grenade Launcher attachment a while ago, and my m16a4 constantly has the underslung rail attached to it, but I can't seem to figure out how to actually use the grenade launcher.

I have looked in the control menu in the options, and it lets me set a key for using underslung weapons, but when I use it it simply doesn't do anything. (also the default hotkey for it is "3", which overlaps with the medkit.

How do I use underslung weapons?

As you already guessed, underslung attachments replace your primary kit, just like the normal grenade launcher would, which would be, as in the example of the assault class, the medkit.

You switch to your underslung attachment by pressing what you normally press for the medkit, you will then switch to your grenade launcher. You can verify that by observing the slightly raised barrel stance of your rifle.

If you haven't chosen the underslung rail as attachment option, then you will access to the grenade launcher as a separate gun, which will cause it to take longer to switch to and back.

For more information, take a look at this Wiki page.