LIDS is located in MIT's Building 32, also called the Stata Center:
The Stata Center is comprised of two towers: Gates and Dreyfoos. LIDS offices are on the 5th, 6th, and 7th floors of the Dreyfoos Tower.
To get to LIDS it is important that you take the Dreyfoos Tower elevators, not the Gates Tower elevators. (The Gates elevators will take you to a different Lab and the two towers are not connected at every floor.)
The most direct way to get to the Dreyfoos Tower elevators is to use the building entrance pictured above. This entrance is on Vassar Street between Massachusetts Avenue and Main Street, but much closer to Main Street. Once inside you will see the Dreyfoos Tower elevator bank:
If you are looking for LIDS Headquarters, take these elevators to the 6th floor. From there, go straight through the double doors on the far left and then make your immediate left. LIDS Headquarters are at the end of the hallway.
If you come into the Stata Center from the entrace at the corner of Vassar and Main Streets (instead of at Vassar between Mass. Ave. and Main Street), you will see this when you enter the building:
From this entrance you will need to walk to the other end of the building to get to the Dreyfoos Tower elevators. Just follow the signs: