On my Android device I click on alarm (when entering a new task) within x minutes or ~1 and make sure that the checkbox is checked.
But, not only does it fail to alert me on the desktop browser (with Toodledo active), it fails to alert me on the same Android device as well. AS if the alarm checkbox was not checked.
Please check that in the settings section of our app under "Notifications" that you have alarms enabled. If this still doesn't work, please create a support ticket so that we can look into it in more detail.