Can I remotely launch an app on my android phone? I have an HTC One S but I don't know where it is. I remotely installed 'Find My Phone' on it via Google Play Store. Now I want to remotely launch it. How can I do that?
Although there is not a general solution for remotely launching apps, here is a solution that applies to your situation:

First, install AndroidLost from the online Google Play Store.


this is the app you can use to control/wipe/access info about your lost device, yadayada


Give it a little time to install to your device, then install AndroidLost Jumpstart from the online Google Play Store as well


  This app will wake up the registration process on the androidlost app
  when ever a phone call is made, an SMS received, battery is low, a
  package is added, removed or changed.


So you have to install an arbitrary app from the Play Store as well to activate the service, but you can then go to androidlost.com to access functions to control your lost device! Including ringing an alarm ;)

EDIT 2015-01-29: Jumpstart won't help on a device with Android newer than 2.x, as in the description you can read: "Please note that google has changed the android API so this app only works on android version &lt; 3."
So not possible with a phone with 4.x to register AndroidLost later, without the physical possess of the phone.