Before you will be able to process Invoices in QuickBooks, you will need to do at least some configuration in Medusa QB. This is done under the Configure menu (along with possible additions/changes in QuickBooks) and is generally the most difficult part of getting things set up. The good news is that once things are configured it should be very simple to process invoices.

The initial Welcome screen or any topic may be viewed by either going to Configure→Configuration Wizard or by clicking its equivalent shortcut icon (circled in red) on the main Medusa QB screen as shown below. 

You may start by selecting Welcome. This screen merely lets you know how to navigate following screens. At any time while in Configuration Settings you may see & select/skip to topics along the left as shown below. Generally you can start with QB Connection and proceed from there (by clicking Next or a topic in the upper left).