Instructions
Obtain a Magmarizer item from Route 214. Pick the Magmarizer off of the top of the northeast cliff face behind some rocks.
Give Magmar the Magmarizer to hold. This is done by selecting Magmar in your Pokemon party, choosing the item option and then the "GIVE" button. Scroll through your item bag until you find the Magmarizer. Select the Magmarizer, and it will be given to Magmar to hold on to.
Trade Magmar to another Nintendo DS game from "Pokemon Platinum," allowing him to instantly evolve into Magmortar upon arriving in the game. Send Magmortar back to your copy of "Pokemon Platinum" to add it to your team.