Download iPhone and iPad link
You need iOS version 4.0 and above to download this app. Go to Settings>General>About to find your iOS version. You can update iOS version via iTunes app on your desktop

Download Android app link.
The app will download in the background. Select notifications and install the app. In case, you get an error that app cannot be installed, make sure to change your seetings.
From Settings-> Applications, enable "Allow installation of non-market applications"