To install Android applications, open Google's Play Store app on the T8. If you've not used it before,
https://droidbox.co.uk/blog/googles-play-store-signing-in-and-auto-updates/ runs you through the basics including allowing auto-updates.
To manually install add-ons in XBMC, first download the ZIP file (often linked to from the page you found describing the add-on) and put it on a USB device or memory card. Insert the device/card into your DroidBOX, then open XBMC. From here, click on the System menu (scroll to the right). Then follow these steps... Add-ons>Install From zip File>Root filesystem>storage>and then whichever device/card you plugged in>click on the ZIP file.
If you find an add-on and the description mentions it can be found in a certain repo(sitory) then open XBMC, then System>Add-ons>Get add-ons>Click the name of the repo>You may need to click something like Video add-ons now>Click the name of the add-on.
In either case, once installed, I tend to exit XBMC (use the power/standby icon in XBMC's homescreen, next to the icon of a star) and then open it again before trying the new add-on.
DroidSticks is a different company from us, but we do share some online resources.
Do dig through the rest of the add-ons for international channels, head to the Media Portal menu in XBMC to see all 80-90 of them listed.