Hi.

1. I want to change the model for a certain item, so that it looks like another item that is in the game. I am guessing I need to export models somehow?

Bonus question:
2. I want to make a mod that has no cells, only adds items with scripts. Which cell is the best for adding "dummy items" to test them out with?

Thanks in advance.

2. Make your own cell. I personally use a worldspace for testing.

I googled and found this:

http://skyrim.nexusmods.com/downloads/file.php?id=247

I managed to extract the .nif file that I want.
I tried editing my item and changing the model to my extracted .nif file, but an error message pops up; saying "Invalid directory."
Now it's impossible to change back to the default model that the item used, and the item stopped showing up in CK as well as in-game.

Any models you put in the game must be under your skyrim directory in Data/meshes/

So I just need to move the .nif file on my computer? Or do I need to import it into the game's data files somehow? (That would be stupid, since the model is already in the file.)
If the first is yes; will the model transfer properly if I upload my mod and someone else downloads it?

You have to place the nif manually in the appropriate folder. It will not transfer to other people online unless you package it in the BSA archive before upload.

I was watching this youtube clip of how to make a custom weapon:

http://www.youtube.com/watch?v=pfJT0VMicFc

When the guy in the video clicks "Edit" he comes to a folder called "Meshes" in which all the game's .nif files are located. -Where can I find this folder?

If you wish to change the model of an existing item (why? o_o) then just extract the mesh and the corresponding texture files you want the final item to have, use nifskope to point the texture files for the newly copypasted and renamed mesh files (if this is even necessary) and then just move your new files into their corresponding folders under the Skyrim\Data\ folder.

If you for example want to change the mesh of an iron dagger into that of a steel dagger, you need to
1. extract the steel dagger mesh (and probably textures)
2. rename the extracted files to have the same name as the old iron dagger files (eg. irondagger.nif, 1stpersonirondagger.nif etc..)
3. move the renamed files into the Skyrim\Data\Meshes\Weapons\weaponname\ folder (and same for textures, just Skyrim\Data\Textures\Weapons\weaponname\).

This should change the mesh of Iron Dagger into that of a Steel Dagger.

I almost got confused myself while writing this. Ehhh.

Adding a custom item is a tad more work, though it's not hard. You should be able to find lots of tutorials for that on YouTube alone.

Here's what I want to do:
I have an item, I made the item by duplicating coal. My item looks like coal.
I want to make my coal item look like a bear trap.
I don't want to change anything, except for my custom coal item's model.
The new model I want my coal item to use is already in Skyrim.

Here are some screenshots:

1:
http://img585.imageshack.us/img585/4143/21361360.jpg

2:
http://img824.imageshack.us/img824/6410/13404245.jpg

3:
http://img42.imageshack.us/img42/5691/31672710.jpg

As you can see in the third picture; there is no folder called "Meshes" under Skyrim's "Data" folder. The only .nif file there was placed there by me. Which I exctracted earlier using BSAopt. If I choose that .nif file, I get the "Invalid Directory" error.

Open up your custom item and the beartrap you want to use the model from to edit.

In the beartrap window highlight the model info and copy it, it should be this - Traps\BearTrap\BearTrap01.nif

Now edit yor custom item until you get to the point in your 3rd screen shot and paste that model info the filename and hit open.

Note this only works because you are using an ingame model, its a little more complicated when you bring in a totally new model.

EDIT: I finally got it to work! Thanks for the help. =)

EDIT: Opps, didn't read @Barney0002's post and said the same thing