Hot-tubs seem to be self-referencing objects. If you're not familiar with them, you can find a tutorial by echo that explains how to fix them here:
http://www.eternal-echo.net/sims/tutorials/self-refs/ .
I've been experimenting with them a bit for my own nefarious purposes, and it seems the key semi-globals you have to import, change, and link are: "4P: Find Next Entrance/Exit Tile" and "4P: Route To Tub."
The working tub linked by KLGFCG has a couple of other semi-globals imported, but I haven't really had a chance to compare mine to see what the differences are. The tub I cloned has been working for 10 sim days straight in a crowded apartment building with no conflicts yet *knocks on wood.*
If you want to see something really scary, read the last page of the novel-length readme included with that MTS2 custom hot tub.
Who in their right mind would include a photo and their personal stats with a download? He must be desperate.