The Winstanley Lecture Theatre is in Whewell's Court, whose entrance is on Trinity Street immediately opposite the main entrance to Trinity College. The lecture theatre should be signposted once you get in to Whewell's Court.
If you have a PDF reader, this very useful map (from the County Council's web site) shows the area and nearby car parks and bus stops. Trinity Street is near the centre of the map, slightly to the left. The railway station is in the bottom right-hand corner.
Alternatively, this other map has Trinity College circled in red. Use this map if you would like to zoom out and see where Cambridge is in context.
The conference is five minutes' walk from the City Centre and bus station, and about twenty minutes' walk from the railway station.
There are cycle racks just outside Whewell's Court to which you can lock a bicycle (at owners' risk!).
The nearest car park is on Park Street, and is accessed via Jesus Lane. There is also a park-and-ride system with a bus stop on Bridge Street, very nearby. (Ask the driver for Bridge Street.)
The nearest bus stop is on Bridge Street and several routes stop there, including the park-and-ride buses and the C1, which goes directly to the railway station. The city-centre bus and coach station is five minutes' walk away.
Cambridge railway station is about 20 minutes' walk away. There are buses from the stop outside the station: any bus marked "Arbury" will stop on Bridge Street, very close to the conference — ask the driver for Bridge Street. There are other buses to the city-centre bus station, a five minute walk. There is also a taxi rank by the station.
It is possible to arrange wheelchair access to the Winstanley Lecture Theatre. Please inform the organisers if you'll need this.
The City Council's website has a page of accommodation listings including hotels, B&Bs, etc. For B&B places where a third of your payment is contributed to Oxfam, ring (01223) 840882.