As is the case with many print servers integrated into routing products, you don't get any controls to twiddle. So if your printer meets the requirements of supporting standard TCP/IP printing using the RAW protocol and it still doesn't work, your only recourse is to double-check your settings.
The admin interface Printer button either shows you the status successfully connected printers or steps you through an on-line copy of essentially the same setup information for WinXP/2000 or Linux OSes that's in the User Manual. If you're running Win98 or Me, the on-line help and User Manual directs you to install a little program from Axis, which should get you hooked up for those OSes.
The only USB printer I had was an Epson Stylus C80, which installed and ran without a hitch. OvisLink says that the 9000VPN will handle up to four USB printers (if you have nothing else connected) assigning each one to its own port (9100, 9101, etc.)