The garmin dongle is meant to be used with a computer, not a phone/iPad. I'm pretty sure it requires some application be installed on the computer to manage the data from the dongle.
As the requirements specify:
System Requirements: Intel-based Mac
OS 10.6.8 or later
256 MB RAM
150 MB free disk space
USB port
That refers to a computer.
It is not meant to be plugged to a iPhone/iPad. Garmin does indeed make iPhone/iPad compatible dongles. However I have only found the 30 pin versions.
Possibly you can use a 30 pin to lightning adapter to connect said dongles to your iPad Mini.
You will not be able to use the USB dongle with the iPad. The drivers required for it are just not available on iPad.