To start a 3D replay, click on "3D Replay" on the left navigation menu
You should now see two large boxes, each with a set of choices, stats graphs similar to the stats page, and a map. From the drop down list in the upper right in the right box, select which one of your friends you'd like to see the simulation with. Then in each box select a run. If you select the same run in both boxes, the simulation will show a replay of that run only instead of a race. A blue button with "Start Race" should apear once you have selected a run in each box. Click it to begin your 3D Replay!
Once your replay is over, you can run another replay by clicking on the blue tab labeled "Choose your run."