Cards are placed in Equipment Slots.
Shoes [1] mean the Shoes have one slot. To insert a card, you have to unequip the equipment, then doubleclick the Card to open a window, there you select the equipment you want to card, and it happens!
However Poring cards are for Armor, so you'll need a slotted armor.
You can't take out cards unless you pay a hefty fine, so choose wisely and plan ahead. If you get a really good weapon, don't put a bad card in it.