I'm a beginner, so I was making a box into a chair. It's a 6-sided cube.

Problem: In-game, in the Buy-Mode, the icon and preview for my object are messed up. The preview shows the entire texture on each side of the cube, instead of dividing the texture up properly.

Not a problem: The object works in the game. The preview is the only thing* that doesn't work.
*Well, I think it's the only thing. There are probably more things that I haven't noticed.

Any help would be appreciated.

This happens to me too when i go into the game to check the item before it's really fixed the way i want it to be. I don't really know what to do, or what I'm doing wrong, but i did find a quick and dirty fix for it. I go into the Thumbnails file (it opens up in SimPe) and find that thumbnail (usually near the bottom of a VERY long list). I delete it and save the file. Then it creates a new thumbnail for it the next time I open the game. NOTE: I don't know if this is the best way to deal with the problem but it works.

That thing happend to me, too, and like bvosim said, it was the old thumbnail from my first version that was still showing.

For me, it worked to just "CTRL-rightclick" the preview icon. The preview icon and preview image are generatad again if you do that, using the current version of the mesh.

I hope that helps,
Svealyn.

73rd survivor,
don't worry, that happens all the time.
And it's not really a big problem. What happens is that the game make a little preview image of your object the very first time it loads it. If you're a creator, that tends to be a testy version with bad textures.
What you do to fix it for yourself is what bvosim suggested. Although you can just go ahead and delete the contents of the the thumbnail folder entirely. This will force the game to make new previews the next time you start it up. It might take a bit longer to load then.

The good news is that if you share your items, this is not a problem for your downloaders, since the first time they load it in their game, it will be the finished mesh, with a nice pretty preview thumbnail to go with it.

Excellent!; I just deleted those files and now it previews properly. (I couldn't find the folder using SimPE, but then I realized it was in My Documents.)

This is kind-of an unusual thing to happen; next time I'm building, I'm only going to preview the object in UVMapper Pro, and then when it's 98% finished I'll load it in-game.