Quintessences are a type of rune, the rarest one. You can only begin to use one at Level 10. Then you unlock a second at Level 20, and the final third at Level 30. This is far fewer than the other runes that you are allowed.
To equip them, once you have reached Level 10 (they will not show up before then) and purchased one from the store, head to your Profile page, then the Rune tab. Left click on the one you want, and move it to the Quintessences section.
Added by: MongooseGeneral Jan 9th 2013, ID#17664