You have a 1:88 chance to get one in a pack in real life, I assume this applies to the video game too.
As you earn more cards, your odds will increase to get rarer cards as you can only earn a certain amount of every card and so as you get more common cards the likeliness you'll get a MR increaases. Thanks to CayMack for this tip.
Mythic Rares are like normal Rares but are even more powerful.
In deck edit, simply select the fourth rarity in the filter which is a browinsh color then add any/all of them to your deck and start any type of match and play one of those cards.
I suggest doing a practice match or finding a boost partner.