NEW: about train tickets. Strangely enough, a return train ticket between Geneve and Lausanne is valid only for one day: this means you should buy a one-way ticket Geneve->Lausanne the day of your arrival and a one-way ticket Lausanne->Geneve the day of your departure.
at the hotel reception, you will be given a card, the Lausanne Transport Card, valid for the duration of your stay, that allows you to travel for free in Lausanne (including to go to EPFL)
Arriving at the Lausanne train station, the easieast way to go to the Hotel du Marche is by walk.
Rather than the lengthy path proposed by
G**gle, try this one:
In front of the main exit of the central station, aim at the MacDonald's, then climb the steep and long Rue du Petit Chene up to Place Saint Francois.
Follow then Avenue du Grand Pont (cross the bridge) and cross the Avenue at the level of the Music Store: follow Rue Pichard until the next MacDonald's, and then follow the steep (again) rue Chaucrau.
Arriving at the top, take Rue Neuve on the left and immediately on the right is the Rue du Pre du Marche. The hotel is on the right, 50 meters later.
To go directly to EPFL from the train station, follow the same path as above, but as you cross the bridge of Avenue du Grand Pont, the metro station Lausanne-Flon is on your left. Take the metro line M1 (direction is Renens-CFF); the trains are frequent, the timetable for week days is here.