In order to download the app outside of Google Play there are a few extra setups you need to take.
Uninstall: If you still have the Google Play version of Where's My Droid you'll need to uninstall it. Best to do that by going into Where's My Droid, tap the menu button in the top-right, and select "Uninstall".
Allow Unknown-Sources: This can be done in your phones system settings, but may vary depending on your phone. There is a great post here on Android Central. You can also do it by going ahead and downloading our .apk, and the system should prompt you on what to do.
Install the app: You can now proceed to our download page and download the app to your phone.
Disable Auto-Updates: By default Google Play will try to auto-update Where's My Droid. You need to disable this. Go to the Google Play Store and search for "Where's My Droid" and select the app. Press the menu button in the top-right and disable auto updates.