Microphone jack...

I switched over in April and I consider myself pretty advanced but there's one thing thats got me - where did the microphone jack go? Do I have to use a USB one?

I know there's meant to be one, but I've plugged it into all the similar shaped ones and nothing happens...

No worries, the microphone is built-in.

There should be a 3.5mm port with a label that looks something like a circle with two triangles pointed inward.

That one is the Microphone port, but like SpindleX said, there is a great microphone built-in. It is to the left of the iSight camera.

oh of course i know about the built in mic, but my external is more accurate and i can pull it closer to me and stuff.

Yes, I found that irritating, the Microphone port needs a powered input (You can't just plug a mic in), you need some sort of amp in between the mic and the port.

I gave up and brought a USB sound adapter for a tenner, it seemed stupid having an amp set up for Skype calling

It amazes me..... what world do Mac designers live in???