From Barkerville: follow the Cariboo Waggon Road south to Summit Rock (about 3.4km from Barkerville). Powderhouse Trail access leaves the Cariboo Waggon Road to the South across a meadow. Do not take the first right turn at the far side of the meadow (that is the ditch trail). The Powderhouse Trail turns right after crossing the ditch then turns left again to go uphill.
This is a very steep trail. It leads to the summit of Bald Mountain which is a nice flat alpine area from which several other trails may be accessed.
More detail can be found at the Friends of Barkerville website: http://www.barkerville.bc.ca.