I downloaded and installed the plug-in "The Hall of Doors", but can't find it. Can anyone tell me where to find it? ???

Have you taken a look at the readme file. Most Mods I have tried explain how to get started there. Good luck

No information as to where isn't in the readme.txt. ???

I've never used this mod but I just had a look at the readme. It's a very good readme compared to many I've seen. There is a quest to figure out how to gain access to the Hall of Doors. The author (theKettle) posts a spoiler at the end of the readme that says:
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SPOILER
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"If you really can't find the Portable Door, here are directions to it:
Enter Addamasartus (the smugglers cave opposite the Seyda Neen silt strider). Go through the first door, and head right down the steps. Keep going straight into the water at the bottom of the steps, and you'll find a little cavern, with a corpse lying next to a sabre underwater. Check the corpse to find the portable door."
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I think the corpse has a ring that, when equipped, gives you the portable door. The corpse may already have the door in it's inventory. If so:
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"Using the Portable Door:
To use the portable door, you must first drop it on the ground. Move around to look at it, and simply click the 'activate' key (Spacebar by default). This will transport you to the Hall of Doors and return the portable door to your inventory."

Thanks!! =D