Monitoring a website is incredibly simple.
Step 1) Find the Hosts link on the left side of the management portal
Step 2) Click on the "Add Host" link on the top
Step 3) Fill out the form. Provide a name for the website to be monitored, provide the domain name for the website, select the locations you want your website monitored from, and click on "Add Host".
Once the host is added, you'll be greeted by a rocket launch, and then get redirected to see your host!