I've been having this problem of Installing third party APK's on my Galaxy S2 ICS. The Problem Unable to click the install Button to install third party APK's even after removing security restrictions to do so. Cause For some reason in ICS 4.0.X the APK install window conflicts with running Screen Filter/Dimmer overlays... So you cannot click the Install Button... Solution Max out screen brightness and Disable any Screen Dimmer/Filter programs including some Battery Saving programs (ie. Juice Defender Ultimate) and then proceed to install the APK. Also to note that Paid Version of Lux Auto Brightness (which is also a dimmer) does recognize the problem and removes its overlay during a install... Smart eh?