Have you got sane, and xsane installed? The hplip driver has a setup program you will need to run also.
Some distros need extra things configured, such as which groups are allowed to access the scanner.
From this link -->http://hplipopensource.com/hplip-web/index.html
you can find all the necessary information for scanners, using setup, and distro specific instructions.
Hope this helps.