i want to create a weapon but i just want to use a default .nif file but it says invalid directory

I have already extracted the meshes for it but i don't know if it is supposed to be in a certain area or not

and second for DDS is a lot of tools and stuff and i also cant find any DDS files to use so would i need to install one is is there something else i need to do

Best way is to find the entry for the sword that originally uses the NIF file you want to use, edit that entry and give it a new EditorID. When you go to save, it will ask if you want to create a new entry. Say yes and you'll have a duplicate of the original weapon with your new changes.

Meshes pretty much need to be in exactly the same place as they were in the BSA, unless you plan to alter the mesh itself.

Easiest way to do this is to always extract more than one Mesh from the BSA. If you extract a single mesh, it will often just give you the mesh itself. If you extract more than one, it will usually create all the appropriate directories for you from the "meshes" directory on inward. Saves you from having to create those directories yourself.

Are you sure you are not talking about the Icon for the weapon ??? - the NIf can be in pretty much any folder - but the icons have to be in the Icon folder and the game will give that error if you try to assign a Nif file to the Icon instead of the icon texture !

Thanks was making it a lot harder then i need to

True for the most part. But if the person is trying to use a vanilla NIF file for a new weapon, then when you extract it you'll want it to be in the same place as it was in the BSA. That way you can still use the one in the BSA and avoid having a useless duplicate that has to be included with the mod.

Also, it ensures that someone doesn't point to a file that's not even in the meshes directory, which I think Oblivion also chokes on.