gnome-help-browser only supports the mousewheel on the scrollbar.
Everywhere else it does not. See
http://www.inria.fr/koala/colas/mouse-wheel-scroll for configuration
instructions; the short answer is that you need to have the Pointer section
in the /usr/lib/X11/XF86Config look something like the following (works for
me on Netscape with a Microsoft Intellimouse w/Intellieye, anyway):
ZAxisMapping 4 5
We are doing active development on a replacement for gnome-help-browser;
Nautilus or Mozilla can be used for now, and a fast standalone browser will
appear when we move to GNOME 2.