The files that you have downloaded have to be put into certain folders in order to work, below is a list of these folders. This is all providing you have kept the default folder that maxis suggested.

Skins: To work in the game:
C:\programfiles\maxis\thesims\gamedata\skins

          To work in simshow:
C:\programfiles\maxis\simshow\gamedata\textures

Walls:
C:\Programfiles\maxis\thesims\gamedata\walls

Note: It's useful if you haven't already to get Homecrafter, which will allow you to add different descriptions to my walls and floors, and also allows you to choose your own price. This can be downloaded at the Sims official website, go to my home page to get the link.

Floors
C:\programfiles\maxis\thesims\gamedata\floors


I hope this helps with any problems you may have, if you still can't get that Sim to wear that pretty little number that you've downloaded or get the lovely pink tiles into the game then email me with your problem and I'll see if I can help.