Obtaining this new Pokemon is not as easy as you might think. Here's our guide to evolving Magneton and getting Magnezone.
According to Magnezone's Bulbapedia entry, once you obtain a Magneton; you will need to LVL it up within a "special magnetic field".
In the past, these special fields have been certain areas of mountains such as Sinnoh's Mt. Coronet.
We are still yet to know what part of the map we will be able to find this special magnetic field, so stay tuned!