Then you probably haven’t imported your KSM account to the extension.
Remember that 1 account = multiple wallet addresses. So if you have an account on Polkadot.js. extension, you basically have a wallet address for every supported network. You can change the chain to display the address format that you need (DOT, KSM, CFG, AIR etc.). Accounts and wallet addresses are explained in this post.
If you still can’t manage to solve the issue, please send me a DM in Telegram as I would need screenshots of what you’re seeing so I can help you. You can find me in the Official chat.