The latest version of Todo doesn't automatically show up in my updates, can I still install it?

Yes. Search for Appigo Todo in the App Store, click the $9.99 button, then "Buy Now". It should prompt you for your password. As long as you use the same iTunes account you used for the original purchase, the App store will recognize that you've already purchased the application and allow you to download it again for free. Here are screen shots demonstrating the process.