What Mac do you have? it needs to have an audio input.
Get a cable to plug your keyboard's audio output or headphone output into the audio input of the mac and record it that way. Depending on the keyboard it could have any of the following:
2 x 1/4" mono outputs
1 x 1/4" stereo output
1 x 3.5mm stereo output (earphone).
You'd need a cable that goes from whichever one of those it has to a 3.5" stereo output to plug into your mac.
It's going to cost you one way or another but a cable will be a few £ at most, where as a full midi/audio interface, even with eBay is going to cost a bit.
Once you have the ability to plug the output of your keyboard into your Mac, you just need to use some software to record it. Garageband would be your best bet if you already have it, if not, Audacity is a free download online and you'd literally just need to open it, hit record and start playing. It offers nothing in the way of sequencing but if you were literally just recording you playing your keyboard live, it shouldn't matter because you can edit out any silence from the beginning of the recording after it's recorded and then save to WAV, MP3 etc...