Click you active invoice. (Could be in Archived invoices)
Click pay invoice. (This will take you to paypal or google checkout depending on what you chose)
If you have paypal, login and pay, same as with a google checkout account.
If you dont have paypal or google checkout below the login box should say something along the lines of "Dont have paypal use credit card or bank account)
If you having any problems with these instructions email support@mainserv.us