Park on the east side of rt 3 - in the flume lot, there is a tunnel underneath Rt 3 that hits the trail. There may be another entry point on West side but I have not used it, good luck and have fun!
I assume you mean in winter. I parked at the Flume Visitors Center, walked up the bike path for about two minutes, and the Mt. Pemigewasset Trail trailhead was on the left (the trail goes under 93). I believe it's 1.8 miles from the trailhead to the summit via that route.
There's another trail that leads to the summit from the south, but I don't recall there being a plowed parking area there last winter.
I enjoy it as a loop, except for the walk on the road. I wish the two trailheads were located together. I've heard that the southern loop is a bit more difficult to follow, but have not found that to be the case when I've gone. Others say that the top end of the southern route is pretty steep, but I've not found it overly-so. Nice views to the valley from the summit.