An original classic 6, you'll need an audio interface to get sound in and out of logic. That high pitched squeal is probably feedback, your mic on your computer coming back thru the speakers. Same can happen with headphones I've heard...
If you're only wanting to use the motif 6 as a controller, and only use the sound sources of logic, then MIDI over USB is all you need...
Might I suggest finding at least an ES series Motif or later? Over USB, the Motif ES or later is a nice controller for Logic, you can start, stop, arm for recording... etc from the Motif itself. The original classic Motifs have a Logic mode but far from useful... The ES series are cheap secondhand...
And...
The ES had the ability to do digital audio over mLAN... FireWire. Really cool... The XS had something similar over FireWire too...
Check out motifator.com for everything Motif...