Dev Mode on the XBox is a very very very limited application. Think of it like a dual boot situation for XBox. You can reboot your XBox into Dev Mode, BUT it is a whole separate XBox install. You can only put files onto that small partition if you will. It DOES not have any access to your retail install partition.
It’s meant to create small games, or setup emulators. You cannot use it at all to modify a retail program or any of its files. THUS, there is no way to install addon’s into your MSFS install on your retail partition. And before you get the idea, you CAN’T even install MSFS onto that dev mode partition. The partition you get is WAY too small to install MSFS even if you could figure out someway to get to the installation media. I mean, look at your XBox in dev Mode. The store, the links don’t even work.
There is no documentation on how to get it to work because the whole philosophy behind Dev Mode was to create a developer playground that was wholly separate and distinct from your retail area. What you are attempting, it just CAN’T be done.
Look at the announcement for Developer Mode from XBox itself if you don’t believe me. " Xbox retail consoles can have two modes, Retail Mode (1) and Developer Mode (2). In Retail Mode, the console is in its normal state: you can play games and run apps acquired through the Xbox store. In Developer Mode, you can develop and test software for the console , but you cannot play retail games or run retail apps."