The Department of Bioengineering is located in the Royal School of Mines (building 12 on the Standard map below). The best entrance to the building is via Prince Consort Road. 

Maps of Imperial College London:

Department of Bioengineering
Imperial College London 
Royal School of Mines
Exhibition Road
London, SW7 2AZ - UK

Closest Tube Stations: South Kensington and Gloucester Road.