I clicked the Todoist for Android but I couldn't find the app.
please clarify - which app are you referring to and where did you click on the "Todoist for Android" link (and where were you forwarded)?
This page: http://todoist.com/mobile
scroll to: "Need other native apps? Other's got you covered"
then I clicked on: "Todoist Android Native Apps"
It didn't take me to the app, but to a search result instead which is:
we're currently working on our first-party native Android app and the apps listed in the search result are the only (third-party) solutions available right now.
Alternatively, you can access Todois's mobile site http://todoist.com/m through any HTML5 compatible browser on your phone.