The application is available for download in the iTunes store and has been tested on iPhone 3GS and above. You can find the application by searching for "QR Check In". Check that the developer for the app you are downloading is "Simon Keary" as there are multiple similar apps.
The latest Android version is available to download directly from the PhoneGap Build web site. The address for the download page is https://build.phonegap.com/apps/352910/builds. Alternatively if you have a QR reader app you can scan the QR code below to directly download the application.
In order to install and Android applications outside of the Google Play Store you have to enable installation of applications from unknown sources on your phone. To do this go to Settings and then select Security. You should now see an option called Unknown sources as indicated below. Ensure this option is checked. When enabling this option you will see a warning dialog as indicated below. Click OK to enable this setting.