Can I use iPhone earbuds as a microphone on my MacBook?

In short, the microphone won't work as a microphone the way you're trying it, but the earbuds will.

Longer answer...

In order to use the microphone from your iPhone headphones, they must be plugged into the headphones socket. You can see in the images below that the MacBook Pro "Sound" preference pane recognises that they have a microphone and switches the "Built-in Microphone" to the headphones one when they're in the headphones socket.

If you plug the iPhone headphones into the Mic/Line-in socket, it seems to be only set up for a standard stereo 3.5mm jack, and so will use the earbuds (both of them) as microphones — a stereo microphone effectively (though they're not great at picking up sound or well matched to the hardware as microphones, so will require lots of amplification).

Sound preference pane without the headphones plugged in the headphones socket:

Sound preference pane with the headphones plugged in the headphones socket:

Plugging the iPhone headphones into the Mic/Line-in socket shows no change (i.e. looks like the top image) and having tested it, only the earbuds seem to produce any response in the computer (i.e. they're connected but the microphone isn't).

On late 2008 MacBooks and on, iPhone earbuds will function as headphones and a mic if plugged into the headphone port.

This depends - do you have separate jacks for headphones & microphone or just one jack? If you have two jacks you'll have to get a splitter for it to work properly.