From Route 128 southbound, take exit 43A (after the underpass) cross over the highway and turn right onto Third Avenue; turn left onto Prospect Hill Lane. From Route 128 norhbound, the exit takes you directly onto Prospect Hill Lane; which becomes Fifth Avenue where it turns 90 degrees to the right. 200 Fifth Avenue is on the left before the top of the hill.
From Main street Waltham: take Border Road, which becomes Fifth Avenue. There is a gate where they join, and it is closed during morning and evening peak hours on weekdays -- but you can get around it on your bicycle!