Kate answers:-

One of the most common causes of this is when the body and head skins needed for the NPC are missing (its quite likely the NPC isn't entirely invisible, but appears as a pair of floating hands!). If you did not place the bmp, cmx and skn files present in the download zip into your gamedata/skins folder as per the instructions then re-download the zip and add those files to your game. 

The 31 character filename limit does prevent some of our objects & NPC's referencing each other properly, the internal coding refers to a longer name than the skin filenames have once it has been truncated to work under your Operating System. To fix this problem create a .far file for the objects and their associated skins - you can get a free copy of FarMaker which if used with its companion download Unzipper can take files contained in our zips and add them to a far file retaining the original filenames and thus does not break the coding that ties all the various object and skin files together, it also saves you having to think about which parts to add as you can simply take the zip you've downloaded and create a .far file directly from it.

Once you have created the far file add it to either Downloads, Gamedata/Objects or Gamedata/Userobjects folders.

But.... its still not working!

If the object does not have the interactions it should do, check that you have put the NPC's iff file in your game folders along with the object itself. Do check that there isn't a townie or other Sim trying to use the object, this will often cause the item to appear as if it isn't working when in fact it is, its just busy 'serving' another Sim.