To install a 3rd party apk, you should prepare the apk by yourself, download it from the internet using browser built in the Set-Top-Box or using a SD card, USB stick. We use a USB stick and the steps are similar to SD card.
From the Home Screen, click on FileExplorer.
On the FileExplorer page, navigate to USB.
Click on 34B5-16EA on the right column and you will see all the files in the stick.
Click on the apk you want to install and install page will be shown. Click on Install to start.
When finished, you can click on Open to use it or Done to go back to USB storage to install another one.
Go back to Home Screen using HOME button on remote, and click on MyApps, you will see the installed app there.