Look at the very bottom of any HUbPages page and you'll see an affliates tab. Click on that and read through, clicking continue to read further pages.
Once your referral tracker is created you can use another tab at the bottom of every hub page which says "link to this page". So for instance go to your profile page and click link to this page. A seperate page will come up which has your page link that includes your referral tracker code in it - just copy and paste it where you want to advertise.
To monitor any referrals you've made. Go to My Account and in the links at the top of the page you'll see one that says URL Trackers. CLick that and it will tell you how many sign ups you have made and how many hubs those sign ups have published.