There is no way. Missigno (Its Missigno) is a glitch, and can only be accessed in any game with a cheating device. Also, DON'T GET IT, because it will mess up your game in some way. If a roaming Pokemon appears on the map and there weren't any before, avoid it. Also, if a roaming Pokemon is already on your map before, good luck. If you encounter Missigno, RUN.
If you have no Pokemon, and you (using the Action Replay) have the walk/run anywhere cheat, when you battle, you will sometimes send out the Pokemon Missingno.
Action replay
No
no you have to get is from an action replay or go to a Pokemon event
Not Possable
It is Impossible without an action replay
You can't without an action replay.
Missingno is not avalible in Pokemon Diamond without help of the Action Replay.
In Diamond & Pearl? Impossible without migrating, trading, or Action Replaying.
If it shows up as a question mark it is the missind Pokemon, Missingo. Missingo really isn't a Pokemon but it is a glitch Pokemon.
you trade it or use action replay
Pokemon modefier from action replay.