Originally Posted by CrazyTrainer
Please help I don't where they are.
Are you aware of the "Swarming" Pokémon?
If not, then read this page here
Magnemite is also a Pokémon that occasionally Swarms on a certain route, in their case in the grass by the Fuego Ironworks.
(you have to Surf north-west along the river going under the bridge, by the Valley Windworks, to get there).
The Swarming Pokémon changes daily, and you can find out what the current Swarm is by checking a TV in a house that has one, or by talking to Dawn/Lucas' sister in Sandgem Town.
Check back everyday, and hopefully she will mention that Magnemites have appeared around the Fuego Ironworks.
You can also change the Swarm by setting the DS' time to 11:59pm, then quickly loading up your game and letting the day roll over to the next.