You want Warcraft to magically recognize the model you want to equip your weapon to?
If you set the model to the weapon, OF COURSE it will only show the weapon! Why should it suddenly equip the weapon to the correct place of the model? It doesn't know which model, it doesn't know where to equip and it doesn't know whether the model is a weapon or a regular unit model.
Think before posting, please...
If you want the weapon to equip when picking up an item, then go to abilities, create a useless ability (based off whatever you want, as long as it is completely useless and passive), make it an item ability and set "Art - Caster" to the weapon and "Art - Caster attachment point" to "hand,left" / "hand,right" or "weapon".
If you want it to be there forever, go to the trigger editor and create this trigger:
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Melee Initialization
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Events
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Conditions
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Actions
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Special Effect - Create a special effect attached to the hand,left of [Your Unit] using [Your Model]
There are also enough tutorials and posts about this, so this is what you need to do:
- THINK!!! Seriously!
- If thinking doesn't help, use "SEARCH".