In order to install non-market apps on any Android device, you need to enable sideloading. This is also known as installation of applications form unknown sources. The process for the Kindle Fire HD is a little different than other Android tablets, and even a little different than last year’s first Kindle Fire.
To get into the settings of the device swipe down the notification bar and tap the “More…” button. With the Settings menu tap the Device button.
On the device menu set “Allow Installation of Applications” to On.
Done! You can now sideload apps to the Kindle Fire HD. For an example, check out how to install the Android YouTube app on the Fire HD.