To gain access to these buildings, you must first report to the Reception desk in that building.
Sometimes you will find the room you want inside another one. You will discover which ones by experience! During the free time in Welcome Week it would be wise to visit all of the locations for lectures, tutorials and practicals you are expecting to attend for the rest of the semester so that you feel confident about locating them in the short time between timetabled periods (sometimes less than 10 minutes).
You can find campus maps at http://www.explore.manchester.ac.uk/campus/