Where's My Droid Lite is the only version the successfully supports remote installing from Google Play. This allows you to find your device even if you have already lost it. The requirements are that you have data enabled on the device, active text messaging, and a Google account logged into the device. (Most do as you need a Google account to access the market)
1. First you need to follow the link below to the market.
WMD Lite on the Android Market
2. If your not already logged into Google you need to log-in in the top right corner of Google Play.
3. Once logged in you can click 'Install' next to the app icon.
4. Next, select your device and click install to accept the permissions.
5. Give the app a few minutes to install.
6. Text either "wmd ring" to make the device ring at maximum volume or "wmd gps" to get its current location.