After logging in, first click on your account settings (top right). After that, go to the "Linked services" tab, select WebDAV setup and you'll be able to select a service from the list. Afterwards you'll need to log into your account to link it to your fileee account.
If your service isn't already in the list, you only have to enter the special WebDAV address and your access data. The addresses can be found at the respective supplier.