First try Google Play: https://play.google.com/store/apps/details?id=com.xensr.xensr
If that doesn't work, try the following to install the app manually. First, download the app onto your phone:
version 2.1: http://dl.xensr.net/android/xensr-2.1.apk
After downloading the app onto your phone, press home and tap on the All Apps icon to get to your app drawer. For most Android phones or tablets, a Downloads app should be in the list. Tap on it, and find the apk you just downloaded, and tap one more time to start the installation.