Turn left out of the rail station. You'll find the bus stops a short way along, on the left-hand-side of the road.
'Citi 1' and 'Citi 7' buses run to Addenbrookes Bus Station. (If you'll still be travelling there from July onwards, 'Uni 4' will be re-routed to run via the rail station, terminating at Addenbrookes):
http://cambridge.com/uk/wp-content/uploads/sites/2/2015/02/City-Travel-Map-page-001.jpg
'Citi 1'runs every 10 minutes, with a 7 minute journey time. 'Citi 7' runs every 20 minutes, with a 9 minute journey time.
As Eddie states, 'Busway A' runs around the Addenbrookes campus on its way to Trumpington. There are departures at 04, 19, 34 and 49 minutes past the hour, taking just 8 minutes for the journey:
http://www.thebusway.info/routes-times.shtml
So there are a total of 13 buses per hour between the rail station and Addenbrookes (with more to be added in July).