I bought a laptop headset from logitech and when i used the headset to plug it into the port it wont play from the mic on the headset every time i try talking through the mic it only receives through the built in microphone on my MBP. how do i make it so my comp will make it come from my headset microphone?
If you are using a standard mini-stereo plug for the microphone, and it is plugging into the line-in port on your Mac, then it is not going to work.
The line-in port is specifically a "line-in" port. It accepts "line level" audio inputs only.
Such inputs would be from the back of your component stereo's tape deck, a component type (as in not built-in) stereo CD-Player's RCA jacks, etc.
Record Players / Turn-Tables and Microphones require a pre-amp if you intend to use them with a line-in jack.
Some microphones claim to be amplified. But, if there is no source of power between the microphone and the computer, you can pretty much forget it.
Does the microphone have a battery? Or, any other form of external power? If not, then you'll need a pre-amp. Usually, it's just cheaper to buy a USB Microphone than a pre-amp.
Myself, I just use a mixer (like a live D.J. would use) with a built-in pre-amp.
Now, if you are using a USB microphone, then you shouldn't be having any trouble.
If you are using a standard microphone, then you'll need a pre-amp.
As for why it works with your desktop, I can offer two possible explanations.
1) Almost all PC's have a Microphone specific jack that has a pre-amp circuit. But, none of the modern Mac's do.
2) Many older Macs do have a Microphone jack. I don't know exactly when it was removed from their systems, but I do know that the old iMac G3 I have has one, and my old PowerMac G3 had one. My newer G4 systems do not, but some of the G4 towers may have (I don't know).
I hope this helps.