Should I wear my uniform to a fast food orientation?

Why are you asking us when your boss already told you what to do?

if your boss told you to wear it to orientation you do so

If the boss says to wear it if you have it, then wear it.

no

Pant and shoes are not a uniform. If your orientation is at the restaurant and they're going to be actually training you, then wear the pants and shoes. If it's in another location or you're only going to be doing classroom training, just wear something neat and appropriate.