Strange. On my PC, all of the default extensions are in \program files\inkscape\share\extensions (the path you mentioned above) and my custom extensions are in the path I mentioned in my post. What version of Inkscape are you using?
Ok, your version should be good. Look at Edit > Preferences > System > User extensions to find the path where you’re supposed to put your new extensions.
OK, I am even more confused now. I installed the elliptical box maker script and it (and the laser tools category) show up fine in Inkscape, but STILL cannot get the TabbedBoxMaker script to show up!