You can export the texture as an .obj??? Whenever I try to export it, it only gives me the option of .png and two others that I've never seen before...

The .obj format is for models. This means only the model can be exported as .obj
Then you can import the OBJ into Blender.

As for the textures, they can be exported as PNG, TIFF and TGA, however PNG is recommended.
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How do I open the texture.png in blender? It doesn't show up any where after I import it....

Do you mean that you imported the model into Blender and the textures didn't appear?

If so, press N, and at the right side that appears, look for the option "Textured Solid". Check it and textures should appear.
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I pressed textured solid, but it just turned it PINK! Did I do something wrong, where are the textures that came with the model?

In blender it won't show the texture unless it's in texture mode. Right click on the model, and at the bottom you'll see a little box which says "Object Mode". You can access edit mode, texture paint etc. there. I don't think it's currently possible to convert/open an nsbtx file without using the leaked SDK.

Actually, I think when creating an NSMBD in MKDSCM there is an option to create an NSBTX. Will check, but not sure if replacing the NSBTX actually really matters.

Yes, just checked. It does have a "Create NSBTX" option.
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hey look, I did a thing

Wait, with the texture paint mode can I create my own texture to use in game? Or will it not work since it isn't a .nsbtx file?

Yeah, it's not very accurate, but say you want to paint a patch of grass let's say, 1 shade of green, you can do that. But editing the exported PNGs is a better idea.
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hey look, I did a thing

When I go to texture paint mode, the textures don't show up, and I can't "paint" on the model, when I select the color and press on the model, nothing happens. It just stays pink.... The Texture Files are .png if that helps...

Select the entire model, click the cube on the left side, than select image UV editor, add texture.
You might have to UV map it or remap it if it came with a .mtl file.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

I went to the uv/image editor, but I don't know how to add the texture, do I just press "open" and then find the .png file?

Pretty much, make sure it is named the exact name as your model and make sure it's the same dimensions as the normal texture.
If you're lucky you won't have to do any UV mapping, and really beyond that I don't really know much else...
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

Would it be easier to create my own texture, export it, and then put it in the game? IF SO, How do I create my own texture?

Update: I uploaded the .png file, and it did nothing to the model

You can't. You must export the NSBMD to OBJ. The program will save the model with the textures. (Use MKDS Course Modifier)

I got the textures to work on the model, but The textures for the head are WAY to big!! And the ones on the body aren't... right, the textures don't cover where they are supposed to... like, the arm is on the torso, etc. How do I remap it?

You might be in luck, try using this lovely tool we have here to open Mario Kart and edit the nsbtx. Make measurements and see if you can find out where everything is.
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"I strikingly thought you would know how but it clearly seems you have no knowledge of hacking" - MiiU 2017

I tried importing my textures over the in-game textures, and then exporting the textures as png using that tool, but It didn't work, nothing changed...

I have never had a problem with anything to do with textures. If you've done it all right, I don't see how you could be having so much trouble, what is your process?
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hey look, I did a thing

OK, I have fixed all my texture problems, the only thing I need to know is... How do I export the textures with the model? when I try to export the model and import it in mkdscm nsbmd creator, I get a ERROR message saying 'Invalid/Argument= Value of '0' is not valid for 'selectedIndex' Parameter name: SelectedIndex..... what happened! I had the textures on the model when I exported it?