The word 'emblem' has a couple of meanings: 1: special design or visual object representing a quality, type, group, etc.; 2: a visible symbol representing an abstract idea
If you are referring to Fire Emblem: Blazing Sword, you can get it from a bonus disk that used to come with Mario Kart: Double Dash. You can also get it by game shark codes or hacking.
I had the same problem but i solved it. There is no way to activate it Marth will just be able to open any chests and doors without keys. Just walk up to one and try it. Hope i helped