Enter it as shown. That is the correct prefix for Thumbnailer. My other question, and please excuse me for asking, but this has happened many times, did you perhaps download the Essentials mistakenly? Often we'll hear that a serial number isn't working and the wrong product is being installed. If that isn't it, leave us all your info on a ticket at the Contact Us link (top of the page).
PS: Yes, you can run it on all your computers.
Embroidery software and designs for Mac and Windows.
"Embroidery for the Rest Of Us!"