jaryr's directions from Toronto via Hwy 89 are okay, but going through Alliston is a pain. A faster way is to take 400 north to Hwy 9 and take Hwy 9 east to Airport Road (Regional Road 18) and head north on Airport Road until you hit Hwy 89. The Bavarian Outpost is on the south east corner of Airport Road and Hwy 89.
Another option (my prefered from Richmond Hill) is to take 400 north to Hwy 9 and take Hwy 9 east to Hwy 51 (Regional Road 51?) then head north on 51 until you hit Hwy 89 and east left on Hwy 89 to Airport Road. This route usually has the least amount of traffic.
For those unfamilar with the Mansfield Agreement Forest (Dufferin Forest), it is just north, up the hill, from the Mansfield Outdoor Centre on Airport Road. As you pass the Outdoor Centre (heading north) and once you reach the top of the big hill, the entrance is on the east side of the road (signed--not hard to miss). Once you reach the entrance don't go straight ahead into the lower parking area but head right (south), up the gravel road (parallel to Airport Road). We park by the gate at the top of the hill, the gate may or may not be open.
See you there.