Most likely, you need to install the drivers, which are available in the "Related Downloads" section of our Get It page. If the red light (LED) on the USB Key does not illuminate when you insert the key, this usually indicates that the drivers are not installed.
If it red light does not illuminate after installing the most recent drivers and restarting your computer, then the key itself might be damaged. This is quite rare unless the key has been physically damaged. If you think the key might have stopped functioning, it is a good test to install the USB Key edition of Isadora and the latest drivers on a differenet computer. If you still can't get the USB Key light to illuminate or Isadora to register, then your key may have stopped functioning. In that case, you should contact us by clicking "New Ticket" on our Help page.
Finally, make sure it is the key that you bought from us. All Isadora USB Keys have the letters ZOMKQ written on them. If you don't see those letters, then it's not an Isadora USB Key!