There is an intermittent problem with the OS X Help Center or Help Viewer application, where the links in the navbar stop working. Turns out the fix is to Quit Help Center, then throw out the file

com.apple.helpui.plist

in

/Users//Library/Preferences/com.apple.helpui.plist

Making the font size larger (since they start out in mouse type) is what causes the bug to appear.

I've found that the help files work pretty well in a browser, however.

Thanks to the people on this forum thread for the answer.