Directions to CERN
On the CERN website you can find directions to CERN.
Transports Publics Genevois (TPG) provide good coverage of the area. If you are staying in a Hotel (not the CERN Hostel) you will be given a complementary travel card. Tram 18 takes you between CERN and the City Centre with a 'Billet Tout Genève validité 60''. Tickets can be purchased from machines at each stop.
Directions to the airport from CERN
To get to the airport, you can use the CERN Shuttle Service (circuit 4), which leaves from in front of the hostel.