Here are the steps for setting up a replacement Fitbit Tracker:

Right click on the "Fitbit Service Manager" icon in the system tray application and choose "Account Setup" (Windows), or double click on the Fitbit application in the Applications folder (Mac).

The setup process will then begin and you'll see a screen that asks you to register. At this point you just want to log in with your existing account. You'll see a link on the page: "Already have an account? Log in". Please click "Log in", enter your existing account information and go through the setup process: plug in the base station, place your Tracker on the base station, wait for the battery icon to disappear, and press and release the button on the Tracker then click on ready. The Tracker will show a series of numbers for you to type in. This makes sure that the Tracker you are looking at is the Tracker that the software is communicating with.

Your new Fitbit Tracker will now be linked to your account.