In order to link your Bonelab installation with mod.io you must have a mod.io account and be logged into the mod.io website.
You can login to the website here: https://mod.io/g
Once you are logged in, you can enter the code given to you in game on this page: https://mod.io/connect
Simply enter the code displayed in game in the box provided (see screenshot below).
If you are still having issues, please open a ticket.