Getting those items requires very good luck! Because of their rarity they get a server wide message when a lucky player receives one.
To help explain the whole process it should look like this; first you get a Kachua Key, which are given as a bonus when buying some items in the Cash Shop (e.g. Infinity Potions). Then with your Kachua Key you summon a treasure chest NPC to turn in your keys. When turning them in there is a small chance to receive an Advanced Refining Certificate Envelope. Those envelopes give one certificate which are named either Armor or Weapon and also will show a + number on it. Once you have the certificate you can speak with an NPC behind the Blacksmith Shop in the center of Prontera to instantly raise an item which matches the certificate type to +10 to +13 depending on the + shown on the certificate.
Hope that answers your question and gave a bit more detail for others who might be wondering how it worked too!