The in-app Help Center searches but does not open links.

The most common reason for this kind of problem is a corrupted plist or cache file.

To fix this, in the Finder, click the Go menu, and choose "Go to Folder"

In the dialog that opens, type ~/Library/Preferences and press Return.

This will open your user Library folder (which is hidden but can be "permanently" unhidden) and inside it, your Preferences folder.

In the Preferences folder, locate and delete the following:

com.apple.help.plist

com.apple.help.plist.lockfile

com.apple.helpviewer.plist

com.apple.helpviewer.plist.lockfile

Then go back to the Library folder, locate and open the Caches folder.

In the Caches folder, locate a folder called com.apple.helpd. Inside it delete Cache.db andHelpcache.plist. Do not delete the Generated folder (it contains the Help files for all of your installed apps.)

Below the folder called com.apple.helpd, there is another folder called com.apple.helpviewer. Delete Cache.db from it.

Restart your computer. Help Centre should now work properly.

Things like this happen with different applications (Help Centre is an app) sometimes. You can follow a similar strategy with other applications too. One common one is Microsoft Word, its has similarly named cache and plist files that can get corrupted and can be deleted.

Another way to accomplish all this and more: download (from Macupdate or Versiontracker) a utility called Onyx, and run it. When it starts up (skip the disk and Smart checks at the beginning, for now), click the Cleaning icon. In this section of Onyx you can tell it which caches to delete. I suggest you checkmark everything in System, Users and Fonts. Then restart your computer. Some applications will be slow for a while as they have to rebuild their caches, but doing this can repair a lot of strange behaviour. You can schedule Onyx to run regularly and do its work automatically.

Good luck.