I suggest you try it this way. In your Applications folder there should be a Hewlett-Packard folder, and inside this is HP Scan Pro. Double-click on this to open it, then with your document in the scanner (and the scanner switched on!), click on New Scan (in the top left-hand corner). This should make the scanner do a preliminary scan. You can then make various adjustments for the final scan. Let us know if this works.
--Richard