Make sure you have followed these steps:
1. Download your license file from the order confirmation email. (Note, the link is not a webpage – depending on email client it might open a blank page. But there is a file there. With some email clients you should right click the link and “Save As..” or “Save Target As…” and the license file will download to your computer). Save this file somewhere safe, for example to your Documents folder. The name of the license file is something like “[your name].license”.
2. Open MIDI Guitar/MIDI Bass and click “Apply License” when the interruption dialog appear (it interrupts every two minutes unless when you play a synth, unless its the Test Piano). A file dialog will pop up. Now navigate to the file directory where you saved the license file and select the license file.