Kashuon is south of Bafsk. Use this map to help find your way there:

GameFAQs: Final Fantasy II (GBA) World Map by ReyVGM

Did you do the quest with Josef to get the Goddess's Bell though? I'm pretty sure you have to do that after Minwu leaves.

Well, if all else fails, try Gamefaqs:

GameFAQs: Final Fantasy II (GBA) FAQ/Walkthrough by Action

Final Fantasy I & II: Dawn of Souls FAQs, Walkthroughs, and Guides for Game Boy Advance - GameFAQs