prep - as this wipes your petit computer save with a save that only has MM2PTC you might want to back up the old save in the following ways:
back up petit computer app to your sd card in System Settings. you can also use the msetforboss tool to dump the save itself "dsiware.sav" beforehand. erase dsiware.nds afterwards.
you can also back up your nand if you're really paranoid
this msetforboss app is configured for the US version you may have to alter it with the python script to get the 
appropriate title id for europe. not tested!

1. put dsiware.sav and MsetForBoss.dat on the root of your sd card. 
2. run the MsetForboss app and choose inject dsiware.

done