Go to /[NAME] to view a tracker. If the name does not already exist, you will be able to create the tracker. If it does exist, you will be able to watch the tracker and, if the room creator gives you the passcode, update the tracker as well.
If you have been given a passcode, you can enter it in the "Settings" panel by clicking the button in the lower-right hand side. There is no indication when you've entered the correct passcode, but you'll be able to update the tracker successfully if you did.
Remember the passcode when you create a tracker. If your cookies clear, you will not be able to update the tracker without putting in the passcode.