There isn't a bus from Milwaukee that goes directly to SFGAm.
The best route I have been able to find so far lies mostly on Amtrak or MegaBus. Both leave from the Milwaukee train station and will arrive at Chicago Union Station. You can then take Metra's Milwaukee District North line to Libertyville. That station is about 6 miles from the park. You can catch a Pace bus to the park then. It will pick up at the train station (not sure if it's in the parking lot or just near it on Milwaukee Ave.) The Pace bus you're looking for is route 572. This will cost about $55 if you use Amtrak. It will be significantly cheaper if you use MegaBus since it can be booked for as little as $3.
I suppose if you take MegaBus, you can cause a disturbance on the bus and get thrown off right around SFGAm

Metra schedule can be found here:
http://www.metrarail.comPace route 572 schedule:
http://www.pacebus.com/sub/schedules/ro ... outeNo=572