Recording from Yamaha P-80, Connection to iMac elusive. Help ! Options

[jodc]

I've only met with frustration in trying to record on my iMac, using 9.2.  I have a Yamaha P-80 with midi capabilities. To start, however, I only want to record audio and make a  basic demo CD. Hope someone can help.
......I have Cakewalk's Metro 5 and downloaded SoundRecorder 1.0 (freeware, very basic), thinking that the only reason I've been unable to record is my lack of experience....I am new to Mac. Here is what I'm doing.

One end of a cable with a 1/8-inch plug is in the computer's Mike port. The end connected to the computer has two stereo plugs, one left and right, for the keyboard's output ports. It appears sound is registered on the software when I play. However, there's no playback. I'm using the internal soundcard. I only get loud static, with an annoyingly loud noise from the left speaker....The Apple Store checked my computer the sound card and port are okay....What do you suggest  ? Please don't hesitate to share even the most basic info. I'm on a steep learning curve...Thank  you, gracias, merci, grazie, obrigado !

[jodc]

Thanks kaboombahchuck. I feel like I've found a lifeline and may have finally spotted the technical problem.....I checked 'sound in' and 'play through.' (That alone was a big help since before I had checked 'external mike' !) Unfortunately, no sound came through the computer speakers after that. But I'm suspecting the plug into the mike port may be the culprit. First of all, when it's plugged in that's when the horrible feedback in the left speaker begins and the sound register jumps all over the place, even without playing a note....
I read--- can't recall where---that 1/8th inch plugs have two lengths, one a bit longer than the other, and that sometime the length (of the metal part) affects a sound connection. Maybe the computer needs the longer version ? My 'short version' is about one-half inch. I believe the longer version is around three-quarters of an inch.
I obviously need to get past this point in order to follow your other wonderful recording tips.

Any ideas about my connection issue ? By the by, I have tried another
1/8th-inch plug (short version) and still had no results..
On the road to recording.....thanks again.....jodc