To filter for all unpaid Invoices, make sure you are in your Invoice dashboard. (Click on the right-most panel from your dashboard view)
Find the DUE column, header, and then select the little funnel to the right of it. This will allow you to filter by amount due. Choose as the options "Greater Than Or Equal To", and then in the empty box below it, click in and put in the number 1. Drop downs will need to be on "Greater Than Or Equal To" and both empty boxes need to have a 1 in them.
Then hit the blue FILTER button. This will now be a list of all invoices for which you are owed money. You can further filter down by a date range, or any other attribute on the job.
Having trouble getting paid on time? Check out our post on dealing with unpaid invoices.