You can download designs one-at-a-time from the OrbitKit UI, but if you're comfortable with working at the command line, there's an easier way:
Export the metadata CSV of your design library (or subset that you care about).
Download the OrbitKit downloader.py file from Github.
Make sure you have a recent version of Python installed on your computer.
From the command line, run
python downloader.py /path/to/mydesigns.csv /path/to/destination
You can control-c to cancel. If you restart, the downloader will pick up where it left off.