If you are seeing a message that says 3 of 3  licenses used or somethiing
similar, this simply means you have used the allotted licenses associated with
your purchase. 

To remove an older licensed machine, to free a slot to license your current
machine you do the following:

Log into your My Account page on the associated website (Song Surgeon, Video Surgeon, etc)

Once logged in, you will find a list of licensed machine in the lower right
section of the page.

You can match this Mac address with those of your machine(s) by opening
Song Surgeon (in licensed or demo mode) and going to Help >> Manual License
At the top of this screen will see the machine ID. This ID will correspond to
those that you see listed in the My Account page.

Delete the ID of one of more of these licensed machines. This will open a slot
for you to register your current machine.