If you want to type in kannada on net, you can install any IME and enable it or type in any website that supports typing kannada and copy paste the content. Kannada Slate is one such website that supports typing in kannada. Similarly, if you want to type kannada in Facebook, you can also follow the same methods.
In the dropdown "Facebook Pages", you should be able to select any page that you manage, like 'Kannadaslate' page in the below example and then click "Add Page Tab".
Once the app is added to a PAGE, it shows as a link on the leftside of the PAGE to all its fans/users, as shown in the below image (for KannadaSlate page as an example).