I'll be assuming that you have arrived at the train station in Cambridge, since train is by far the most pleasant way to travel in the UK. There are direct trains to Cambridge from Stansted Airport and from London King's Cross, which is a short walk from St Pancras International. It is always cheaper to buy a return ticket (and even cheaper if you travel back the same day). It is very important to specify the exact time of your return, since it is more expensive to leave Cambridge before 9.30 in the morning than it is if you leave later. In all probability the person or machine selling you the ticket will *not* ask you this: when you then turn up at the station your ticket will not be valid. The ticket inspectors at the station will generally take the attitude that this is your fault, will not offer either an apology or an explanation, and will direct you to the queue where it will be necessary to pay a supplement of 11 pounds. Note that train travel in the UK is, by any standards, extremely expensive. Trains to Cambridge from King's Cross are easy to find; the one possible source of confusion is that the final destination of the train may not be Cambridge but a place in the fens called King's Lynn or, exceptionally, another place in the fens called Ely. All such trains will certainly stop at Cambridge.
The chances are that I've arranged to meet you at one of the following two locations: (1) Trinity College Great Gate or (2) The Centre for Mathematical Sciences, or CMS for short.
Option 1: Trinity College. Here is a picture of the meeting place. Personally, I always walk from the station - this takes 20 minutes at a moderate-to-brisk striding pace. Most of the route is easy to describe: walk out of the station, proceed in a straight line for about 5 minutes, take a right turn onto a main road and walk down that for about 15 minutes. This will cause you to be in the centre of Cambridge, at which point things become a little complicated. You can either ask someone or consult a map (either bring one, or look at one of the many maps situated around town).
All visitors to Cambridge notice that the station is a peculiarly long way from the centre of town, a fact for which Wikipedia offers the following explanation: "The station was built some distance from the town centre (about one mile (1.6 km) south-east) owing to opposition from university authorities according to legend, but engineering factors were also important".
It is possible to get a bus to the city centre (after that one must walk for about 5 minutes to get to Trinity). The bus routes from the station to the centre are 1,3 and 7. Be sure to check that the bus is going to the centre. This will not always be clear from the destination on the front of the bus. Very occasionally the bus driver will be rude when questioned - this should be completly ignored. Many people have found that it isn't completely obvious when to get off the bus. I'm afraid it is rather difficult to offer advice on that subject - asking people around you or the driver is probably the best option.
One can also get a taxi to Trinity College but this can be a little complicated as there is a pedestrianised zone. The driver should be able to help.
Option 2: Centre for Mathematical Sciences. Despite the address being Wilberforce Rd, you should tell the taxi driver to go to Clarkson Road. The Centre for Mathematical Sciences is the only substantial building (or, rather, complex of buildings) on that road. Come in through the main entrance, which can be accessed along a little lane leading off Clarkson Road. The reception area is at the left at the front of the building, and you can ask for me there. If for some reason I have left my office, there is a cafeteria available. The coffee is rather ordinary, much worse than I can make in my office, so it is perhaps safe to stick with tea.
It is certainly possible to walk to the CMS from the station, and there are some very pleasant routes if the weather is nice. It takes about 40 minutes and you'll need a map. It is also possible to walk along "the backs" to get to the CMS form the centre of town: again, a map is advisible.