When trying to install WebWatcher for Android and the installation is failing with "Can't Open File" error there is a work around to complete the install.
1) On the device you are trying to monitor, open the Google Play Store app and search for "OI File Manager". Download the free OI File Manager App from the Google Play Store from here.
2) Open the OI File Manager App and find the "a.apk" file which was previously downloaded in the Downloads folder. Click the "a.apk" to launch the installer.
3) The installation screen will load the you will be able to now complete the install.
4) (Optional) Uninstall the OI File Manager app from the device.