OS X Tiger (10.4) does not provide a standard way via the UI to add non-standard directories to the Spotlight index. Fortunately, OS X does provide the ability to add directories via the command-line. There are at least a two different ways to add directories to be indexed by Spotlight.

First, you can tag a directory via the command-line using mdimport:

mdimport -f /directory-path

Alternatively, you can edit the configuration file directly and force a reindex:

sudo /Developer/Applications/Utilities/Property\ List\ Editor.app/Contents/MacOS/Property\ List\ Editor /.Spotlight-V100/_rules.plist

Once complete, run sudo mdutil -E / to erase and reindex the drive.

Note: the sudo mdutil -E / is also useful to rebuild a corrupt Spotlight database.