Implementing Upgrades on Website

I have the Upgrades plugin installed in WordPress, and I have Multi-Sites enabled, I have configured Upgrades for the values that I require, however, I am having trouble understanding how to get this to work for users on my website. How can I set this plugin up to work with multiple user accounts on the website and produce a page that will give me the users current credits, and to create a link on one of my web pages to allow users to pay for additonal credits when they need them?

Any help would be greatly appreciated.