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 !