if you go to "change team", if you look just above the two boxes, you will see a button called "all your pokemon". click that button. there, you can choose the pokemon you want on your team and the one to take off your team.
click the tab "Your Account", then click on Item Inventory. Click on the Pokemart, then go down to the mega stone section (bottom of page), then just look through the mega stones to find the Charisardite X