You Have two different options for getting fitment attached to your Private/Custom Catalogs
Option #1 - User Interface
This is a great option for adding fitment to one product at a time and dealing with less than 50 products. For detailed instructions, Click Here
Option #2 - Import csv File
Add fitment by importing a structured csv file into the File Center. This is the way to go if you are looking to add fitment to several products in bulk. For detailed instructions, Click Here