External speaker help?

Please forgive this question if it turns out my ignorance is showing.
I have an awesome set of OLD Altec Lansing speakers that I would love to use on my 27" iMac. I have a mic port and what I assume is a line in. I plugged the speaker into the supposed Line in and nothing. I went into system preferences, sound, output and the only choices are Internal Speakers and Apple TV.

Is there a step I'm missing? Is there a way to make the Mac "see" these external speakers?

Yes, I am a noobie. Somehow I expected the Mac to recognize that something was plugged in. Is this not a line in port?

... and the help screen was no help at all. <sigh>
Thanks to any takers.

You can't attach speakers to a "Line In" port. That port is just what it says it is: it's for a high level sound input to your Mac. The 27" iMac also has an audio output port but the signal level will not be strong enough to drive the Altec Lansing speakers.

You would need a preamp or stereo amplifier between the output of your iMac and the speakers. The Altecs are likely 8 Ohm speakers and need adequate driving power. You can probably purchase an inexpensive set of powered speakers including a low level driver speaker (sub woofer) for around $100. I purchased a set of Yamaha speakers including sub woofer when they were on sale at Best Buy and they sound great to me.