Connecting a musical instrument to the computer

[h_howee]

I'm trying to get audio from a keyboard into my computer. I have a 1/8' to 1/4' adapter, and I can hear the sound from the other end when I connect my earphones to the keyboard, but if I connect it to my computer's microphone port, it doesn't even recognize something connected there. Sound preferences tell me that there are two microphones connected (two built-in microphones), and so does Jack.
How do I get it to detect the keyboard?

I'm using Ubuntu 9.10 64-bit, if that's of any relevance.

[edit]
I also tried connecting this computer to another computer (i.e. Speakers are connected to computer 2, a 1/8' cable goes from the speaker's earphone socket and into computer 1's microphone in). I expected the sound produced by computer 2 to go to computer 1 like regular microphone input would, but it still didn't detect anything and persisted in telling me that there are only two sound inputs on the computer.

(I'm unsure of the terminology; socket, port, line-in, those things. Am I using them right?)

[Enigmapond]

It would be Line in...not Microphone. At last that's now mine works. I just connect to Line in and make sure the mixer is not muted and the volume is up.

[h_howee]

Oh, that must be it. I thought mic and line in were the same thing. I'm using a laptop and I only have one for mic and two for sound output.
I'll go try it out with another computer.

[edit]
Ok, it works when connected to a desktop. Is it possible to get the audio from the desktop's speakers to the the laptop's mic socket, or does that have to be done through line-in too?