Here is the location to add your own fabrics.
I'd suggest you create your own folder to put them in as it will be easier to manage.
When you want to use them in the software go to: Design Settings Menu > Background and Display Color > check "custom colors".
Also a tip: make sure the size of your image for fabric is not too large. Otherwise, it will take up a lot of your computer resources to display and when you start digitizing, it could run out of resource memory.