From Central Station go by subway to the Hostel.
Take the green line north for two stops until you come to the station called Rådmansgatan. Go out the exit marked 'Handelshögskolan' and then follow signs to 'Sveavägen'(name of a street). When you enter 'Sveavägen' go to the left and you are on Rehnsgatan and you will find the Hostel on the other side of Rehnsgatan.