Hello, I've had the same problem I think it's caused by changes to the Arduino IDE. I downloaded the old Arduino 1.6.9 as a zip file, extracted the folder and ran it. It will read your existing preferences file and you can install the Digistump AVR boards. Delete the 1.6.9 folder and run your current version of Arduino, which will have the Digistump AVR boards.
The problem I have now is that it has installed the bootloader when the Digispark is plugged-in but it hasn't assigned a COM port.