my imac wont recognize my input in the system preferences

so, im on an older imac (the one with the big round base, a chrome arm holding up a flat screen) and im trying to hook my guitar into the line-in jack next to the headphone jack. i go to "system preferences" and click the "sound" tab, then click the "input" tab. it only gives me the option for the built-in mircophone, i dosent even recognize that i have a line in plugged in. the wire is in perfect condition, so there is no chance that the wire is faulty. i have tryed it with the 1/4 end of the cable plugged into my guitar, and with out, and ive tryed starting my computer with it already plugged in, and i tryed it while the computer was already running. ive recording many songs on my X-girlfrinds MacBook Pro with no isses...same guitar. same wire, and it worked great.

how can i get the "system preferences to recognize that my guitar is plugged in?!?!?

thank you!!!!

PS this computer was a hand-me-down from my dad, i have no clue what version of garageband i have (not that i think that would matter in this situation) and i do not know what was the last operating system update that this computer recieved