The “as delivered” configuration of your smartphone will only allow you to install apps from the Google Play store. Before being able to install applications from other sources (such as this site), you have to change your device’s security settings. Go to the Security menu item in your device’s settings. You will find the option Allow installation of apps from unknown sources under Device Administration; enter a tick by clicking on the menu item and entering OK in response to the confirmation prompt.
Further information on the manual installation of apps on the Android OS can be found here: