I use Jack quite a bit (and was just about to suggest it as well). Make sure the number of outputs from SC is high enough:
And I think the actual device name for Jack is JackRouter:
THEN - you need to go into Jack Pilot and connect your scsynth outputs to your Logic inputs. Since you are using Jack, you actually don't have to change your Out.ar anymore! So, Out.ar(0 ... ) is the same as scsynth - out1 in Jack. Open the Connections Manager in Jack. On Send Ports click the triangle to the right of scsynth to bring down its ports. Then, click 'out1' once. Go to the Receive Ports, open Logic's triangle, and DOUBLE-CLICK the port you want SC to go into (probably in5?). Both out1 and in5 will now be red. This tells you they are connected, and you should be ready to go.