The most straightforward way to get to Fort Collins from DIA is to take the new toll road, E-470, to get to I-25 northbound. The E-470 on-ramp is clearly marked, watch the signs as you leave the airport. You will need to pay two $1.75 tolls. Once you reach the interchange with I-25, shortly after the second tollbooth, head north on I-25. In about a half-hour (if traffic is good) you will approach Fort Collins. Go past the Harmony Road and Prospect Road exits and take the next one, Exit 269 and head west (towards the mountains!) about 5 miles into town. This road becomes Mulberry St. You will soon come to a large street, College Ave with a Safeway Store on the corner. Turn left (south), and go to Laurel St., where you will make a right (heading west).
As shown on the detailed map, in only a few blocks you will come to Meldrum, where you turn left into the CSU campus (directly into the large parking lot north of the Engineering building). The Physics Department entrance is in the D Wing of the Engineering building, next to the large white liquid nitrogen tank.