Headphone mic help

So i just bought a new headset so people could hear me better through the mic instead of hearing the fan on my laptop through the built in mic on my MacbookPro. I know how to go to system prefrences > sound > then choose either (line in) or (internal microphone). But when i click on (line in) it still stays on (internal microphone). Please help!!!!

I'll take a shot here. Am assuming you have a wired headset? Designed for use with mobile phones... And you are plugging the single plug into the audio "out" port?

You could try something like this splitter, not sure if it'll work for that or not. But, it's a cheap enough experiment.

Most computer headsets have two jacks - one for in, one for out. I don't know if the splitter would really do much for you or not since it is not really splitting the in and out signals, just splitting the overall signal. If you didn't spend much $ on your current headset (or still have the receipt) I would recommend taking it back/biting the bullet and get a headset that has separate in & out connectors.