If you're using the web app, select the Project for which you'd like to display your Completed Tasks (in the left hand pane). At the bottom of the right hand pane, you should see a link at the bottom of the Task list, on the left side, "Get completed tasks". Click that link to view your Completed Tasks; please be aware that you may have to click the link multiple times.
Is it possible to view these tasks on the android app?
At the moment there is no way to view your completed items on the Android app. We will likely be adding this in the near future.
You can add that suggestion in our votebox:
I've posted the issue on the Chrome App store support page. It seems Google should easily be able to tackle this. I'm seeing the same issue occur with a few other apps as well.