There is a checkbox in the advanced settings to turn it on (only works if you are using Chrome as a test agent):
In your main test results page you will get a link next to each waterfall that will let you download the timeline file:
After you have the timeline file, you need to open Chrome’s Dev Tools (F12), right-click and select “Load Timeline Data”:
Yes, you do need Chrome to be able to view the timeline (eventually we may build an online viewer).
If the “Load Timeline Data” menu item isn’t bringing up the file dialog, try checking for a Chrome update. This was just fixed in Chrome 18 (current stable version).