Assuming your WiFi router is willing to use the USB port for a disk, as apposed to a printer, you need to know how the WiFi router is going to present the disk to network. Do you have to pre-format the disk, or will the WiFi router format it for you?
Once you satisfy the WiFi router's needs, then most likely you would connect to the disk as a network shared disk. If the WiFi router is nice, it will advertise the disk via bonjour, and will appear in your Finder Side-Bar, or you will need to use something like Finder -> Go -> Connect to server -> smb://router.ip.address
Again, it is important to know if your WiFi router is going to offer its USB port for a disk drive.