Short version: you probably can't, and certainly can't download it to SD.
Longer version: moving apps to SD was introduced in Android 2.2, improved slightly in 2.3, and dropped in 4.0. Some manufacturers retain it in some of their phones, but I don't know about the Admire. If you go into Settings > Applications (which I think Samsung might rename slightly) and select some downloaded app, is there a "move to SD" button? If there is one, even greyed out, then your phone does support it. If not then it doesn't.
If you have this option, you can use it to free enough space to install Messenger. Not all apps allow you to move them: you can't move pre-installed apps (which is why I specified "downloaded" above), and not all user-installed apps support it, but if you have the option some will. It actually only moves part of the app and does not move app data, and you should not move apps whose widgets you use (the widget will break next time you reboot), or apps that need to run on start-up (also incompatible with moving to SD, as that is mounted too late in the boot sequence). Since apps always download and install to internal you'd need to use this to free enough space to install Messenger. Actually I wouldn't be at all surprised if Messenger itself does not let you move it, as that probably wants to start when you boot the phone.
If you don't have a "move to SD" button then your OS doesn't support it, and without rooting the phone there is nothing you can do.