It is backwards :). The actor attaches itself to a host unit through its (Basic) Event: Events field, even though every other type of object is attached from the host. The easiest way to make a new one is to duplicate an existing unit's actor.
Alternately, if you want to change the model of an existing unit, you can go to the unit in the data editor, scroll through its attached files (e.g. actors, abilities, etc.) until you get to its models, then in the unit model change the (Basic) Art: Model field. (You can do the same to change the unit's death model, missile model, etc.)
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