Making a new Cargo Proxy

[Lean Bear]

I'm making a new cargo proxy for an addon, but when I try and use the addon (a truck), an error message comes up saying something like this:

error loading 'data3d\hangcargo.p3d" (Magic)

I had a look around and found that some vehicles use .p3d's for some reason (eg. CESSENACargo.p3d). So I made one but I don't really know what to do with it

btw hangcargo is the name of the ProxyCargo.

Any ideas?