Press the "Home" button, which is the center button below the main display. This takes you to the BACKFLIP's "Home" screen.
Press the "Menu" button, which to the left of the "Home" button.
Tap "Settings," then "Applications."
Tap the "Unknown sources" option to place a check in the box next to it. Tap "OK" in the window that appears giving you further information about this feature.
You can download APKs to your phone directly by tapping on links to them in your BACKFLIP's web browser; then just drag down the notification bar from the top of the screen when the download completes and tap the download to install it.
If you move an APK onto your microSD card from your computer, you can install it using any of several reliable apps from the Android Market, like the "ASTRO File Manager" or "AppsInstaller."
Although many non-Market apps are safe and reliable, there are some apps that could compromise your phone's security or stability. Don't install a non-Market app unless you're sure that you trust the developer.