How do I add a template to a file location on my computer so it appears in
the Spreadsheet Solutions tabe when I click File New and then click On my
computer... in the right-hand pane?

I've dropped an .xlt to this location on my computer [1]; this appears to be
the location from where the Spreadsheet Solutions tab seems to draw the
templates for its display; however, the template I dropped in there doesn't
appear in the panel.

[1]
C:\Program Files\Microsoft Office\Templates\1033