The only way I know of getting extra textures to show in that plug-in is to click on the load button at the bottom of the list of textures and then direct photoshop to the file.
This is my link to the file loaded from version 7
C:\Program Files\Adobe\Photoshop 7.0\Presets\Textures
I hope this helps.
note these textures are included with Photoshop 7