First are you trying to scan using the scan button on the all-in-one or from the computer? If you are trying to use the scan buttons, you need to install the HP full-feature software package:
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?os=219&lc=en&cc=us&dlc=en&sw_lang=&product=458978#N193 . (If you need to expand the section marked drivers.) Uninstall the printer and disconnect the data cable before installing this full software.
If you installed the basic drivers that came with the Mac OS, you can scan using Apple Preview and Image Capture software. You can also acquire a scan from the Print & Fax > Printer browser. Open the software from the dock or from the Applications folder. In Apple Preview, choose File > Import from Scanner. In the Image Capture program select the all-in-one from the list of devices.
For the Print and Scan. Open System Preferences from the Apple Menu. Click Print & Fax and select the all-in-one. Click the Scan tab and Open Scanner. Then click Scan.
With the HP software you can launch a scan from the computer or from the scanner (but only if the monitoring program is running). You need to set up the HP Utility for the scan task manager.
This site has the full set of steps:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c02456650&tmp_task=useCategory&lc=en&dlc=en&cc=us&product=458978#N2158 .
Sometimes an all-in-one will work for printing but not for scanning due to a damaged cable. Scanning requires bi-directional communication with the printer. Try replacing the data cable. If you use a USB hub or print server, try a direct connection to the computer.
I hope this helps.
Cindy Wells