My hub may be a help: http://casimiro.hubpages.com/hub/How-to-Re-purpose...
In short, you don't add a referral tracker to a hub. You derive a URL with a tracker code embedded in it from a hub. At the very bottom of the page of your hub is a link that says "link to this hub". Click on that, you get a popup, which has a list of all your referral tracker codes at the top (you have one by default, you create the rest). Then just copy the URL below and paste it wherever.
If someone else follows that URL, the hit will be recorded for that tracker code. To create a tracker code, go to your Account page->Earnings->Referral Trackers
At the bottom are two fields where you can create a new tracker. Unfortunately, your tracker has to be unique across all HP, and there is no error message telling you if it's not unique, it just silently fails.