To get to the Mathematical Institute, you can go by bus or by taxi. Using buses is much cheaper (a taxi to the Institute is rather expensive, about 2,500 yen), but taxis are more convenient and allow you to avoid learning how to use the bus system. If it is possible for you, perhaps the best way to get to the conference is to share a taxi with other participants (lowering the cost per person, and avoiding the less convenient bus system).
To take the bus, the trip begins at the bus pool, which is located in front of the station and just to the north of the Hotel Metropolitan Sendai. There you will see many bus stops, marked by rectanguloid posts numbered from 1 up to the 30s. The W7-1 bus goes to Tohoku University and leaves from the post marked with a 9. The W7-1 is not very frequent--it comes about once every half hour, and once every twenty minutes during rush hour. Boarding at the rear door of the bus, passengers take a white slip of paper from a machine located just inside the door. This white slip is used for determining the fare that must be paid at the end of the trip. For this trip the fare is 220 yen, and it is best to have exact change ready. The stop you get off at is about the 9th stop. It is the stop called the Rigakubu-Mae. ``Rigakubu'' is Japanese for ``Faculty of Science'' and ``mae'' is Japanese for ``in front of'', and anyone on the bus will understand this word. About two thirds of the way through the bus trip, you will cross a river. Rigakubu-Mae is the first stop after going up a long hill, about 1 kilometer long.
After taking this bus trip once, you will find it is a convenient way to reach the Institute. Also, you will likely find a fellow participant who can take the trip with you and show you how to catch the right buses.