First you must be in Chapter 5, if your in then great!, second go to the airship and select Bevelle and you'll see no mission there but there is one!

Go to Bevelle and go to the entrance, an clip will come and you'll meet the Kindergartens. Follow them, to do this you must go inside the entrance and turn right keep goin' and you see another path go inside. Now go to the sphere where situated on the ground, you'll watch a clip (of someone telling you something!?).

All you have to do is get through every cloister up untill cloister 20 or 40 at 20 or 40 you'll eventually fight an elemental fiend fight it win the battle open the chest you'll find Cat Nip and other accessory.

Note. Get through all the cloisters up untill cloister 99, and you'll fight the bloodiest boss you never thought of , gu's name is T???A!, founder of Yevon! P.S Have the Cat Nip accessory on to beat him! Good Luck!!!!!