Garageband/Guitar Help!

[CalebWilson]

I just bought and received a guitar cable from Ebay. New.
Using an adapter, I connected it to my computer and to my guitar (well, it's an electric ukulele, but it's mechanically exactly the same as an electric guitar.)

I opened Garageband, clicked "new instrument", clicked "real instrument", and it made a little thingy for my uke playing to register. But the volume bar didn't move at all when I played one of the strings. Or even any of the strings. I looked in the Garageband help, and was somewhat relieved to find that i should change my input source to "Line In". And so I did so.

Still, it did not register my little friend's vibrations. Please note that it works fine in actual amplifiers, even my little headphone amp.

Thanks a million!