You can get to campus by Metro (subway) or by bus.
Metro: red line (T14) towards Mörby Centrum, get off at Universitetet (Map over the Metro system)
Bus: Bus 50, direction Stora Lappkärrsberget or Universitetet, get off at Universitetsvägen

Directions for those staying at Hellsten (www.hellsten.se):
By Metro from City Terminalen/Central Station: take the green line (T17, T18, T19), direction Hässelby strand or Alvik (see map above), get off at Rådmansgatan and then walk.
Find your way with this map

To get to Stockholm University from Hotel Hellsten: take bus 50.
Bus route map