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

 There are 3 files in this set. Black iron stairs with mesh insert, black iron true mesh floor for escape balcony, and a matching black railing to keep the sim from falling off. lol  Place all 3 package files your, "...\My Documents\EA Games\The Sims 2\Downloads", folder.

Then --

Method #1
 If you use Numenor's Scriptorium (http://www.modthesims2.com/showthread.php?t=192613) from MTS2. 
   In this zip there is a txt files named, "914_CTN_HolySmokeIronStairs-Black.txt",  just follow install instructions for adding stairs and place that txt files in the proper folder. 

Method #2
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 this file below the line, to the end of that file. Save that file and close it. 

 Whatever method you use, when you start the game you should havea 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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# CTN_HolySmokeIronStairs-Black
modularStairs 914 
objectID landing 0x002CC58D 
objectID placeholder 0x002CC58C 
objectID step 0x002CC58E 
objectID topStep 0x002CC58B 
objectID bottomStep 0x002CC589 
catalogTextIndex 915 
