No stupid question at all. Think only a few users know about how to manage your parts.
You can't name or organise your parts via MM's UI. You need to use your OS file browser instead.
If you never changed the library files' structure you will find a new part in .../Documents/meshmixer/libraries/parts/user/My Parts
A valid part needs at least three files:
1. the geometry (obj) e.g. "1529319538_00001_Cube.obj"
2. a preview image (png) e.g. "1529319538_00001_Cube.obj.png"
3. a textfile storing the part's definitions (prt) e.g. "1529319538_00001_Cube.obj.prt"
All three files have to start with the same name which by default is a auto-generated number + the name of the object it was generated from (this is the file name the object had when it was added to the scene - later renaming via object browser is ignored). Plus the png and prt files need the "double suffix" .obj.png or .obj.prt.
You may rename all three files the same way. So the upper example would work too if you'd rename it to:
"Something.obj"
"Something.obj.png"
"Something.obj.prt"
This might be nice if you actually want to find the files via file browser but doesn't make that much sense in MM. Dropping a part as an separate object to empty space MM ignores the names of the files and shows them as "MeshSO"+ some index number.
But you can do another thing:
By default there's only a "My Parts" directory in
.../Documents/meshmixer/libraries/parts/user
You can add more subdirectories to the "user" level. These directories appear as categories in MM. If you create a new part now it is added to the last used category. If the last used category is under "default" it is added to "My Parts". If you deleted "My Parts" before MM will create a new "My Parts" directory.
You can do the same (or even delete default subfolders) for
.../Documents/meshmixer/libraries/parts/default
Compared to the "user" directory the only difference is that you can't add or remove a part via MM's GUI here.
In menu:File there's the option to "ImportPartsFolder". This command adds the content of the folder to the last used category
