 I'm including 2 ways to install the stairs, use the one you prefer. 

 First, place the stair package in your, "...\My Documents\EA Games\The Sims 2\Downloads", folder.

Then --

 If you use Numenor's Scriptorium (http://www.modthesims2.com/showthread.php?t=192613) from MTS2. 
   In this zip you will find a txt file for the stairs, just follow install instructions for adding stairs and place that txt file in the folder. 

If you use the old method of adding them to the, "C:\Program Files\EA GAMES\The Sims 2\TSData\Res\Catalog\Scripts\modularstairs.txt". Open that file, add a blank line between the last line in the file and then add the text you will find in the txt file from the stair zip to the end of that file. Save the modularstairs.txt file and close it. 

 Whatever method you use, when you start the game you should have a new icon in the modular stairs section of the catalog, they will work just like the Maxis ones, and all have matching railings that are applied with the rail tool.

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