This library has been deprecated in favour of DigiCDC - Digispark USB CDC Serial Library https://digistump.com/wiki/digispark/tutorials/digicdc. With DigiCDC the Digispark will show up as a COM port and the Arduino Serial monitor can be used normally. DigiUSB requires communication to the Digispark through a seperate application.
I couldn't get the Windows device driver for the CDC Serial Library working. For me it shows up as unknown usb device (invalid configuration descriptor) and this older method works.
While the Digispark is programmed through the micro USB connection, the Digispark will not show up as a COM port, so will not be able to write to the Arduino serial monitor through this micro USB connection. However, it is possible to install a library on the Digispark and and on a windows machine, a windows driver and then to communicate to a separate Monitor application. There are MAC and Linux alternatives also.
This Monitor program is like the Arduino serial monitor, allowing you to send and receive messages to/from a Digispark.
See the DigiUSB→Echo example and the applications in the “Digispark - Example Programs” folder for an example of how to use the DigiUSB library.
On the computer side you can use the included command line tools or more easily the Monitor Application in the DigiUSB Programs folder. The Windows DigiUSB programs are at https://github.com/digistump/DigisparkExamplePrograms/tree/master/Python/DigiUSB/windows :
Also check out the rubygem
The rubygem is a package installable using the rubygem package manager (you wont get any benefit from just downloading the files from github, unless you also use the gem tools to compile it in to a gem and install it.
To install the rubygem, make sure you have libusb 1.x and ruby 1.9.x or ruby 2.0 installed (mac's and many versions of linux come with 1.8 installed, which is no good for this), then open a terminal and run 'gem install digiusb'. You might need to sudo that depending on your platform. Finally, you aught to be able to just type 'digiterm' in your terminal window and it will find the most recently attached digispark, connect to it, and start outputting everything printed via DigiUSB on the device and forwarding keystrokes from the terminal window in to the digispark.
You can get RubyGem from: http://rubygems.org/gems/digiusb