Hi evilnando. I'm a little confused by it all myself, but I did manage to get a model in the game where I wanted it (though not yet a custom one for blender, that's my next task!). I'd recommend looking at the sample called "SimpleScenery". I'd imagine if you copy that sample project, and replace your custom model with one of the scenery models there (e.g. MSFS SDK\Samples\SimpleScenery\PackageSources\modelLib\SampleMyBox), you could get something going. It looks like the xml file there has a GUID for the model. You can generate your own one by googling GUID generator.
There's another xml file in "MSFS SDK\Samples\SimpleScenery\PackageSources\scene" called objects. This lists the objects in the project, along with a global position (latitude and longitude coordinates), and the GUID from above. You can find the coordinates that you want your object to be by clicking there on google maps.
I hope some of this helps, if I get a model exported from blender and into the sim, I'll let you know more!
EDIT: Also I think once you open the project in the sim, you can move the object around to fine tune its position, then save the project. That should save the position in that xml file, though I haven't tried that yet. Honestly I hope this isn't the right way, since it's quite an effort to add an entry to the xml file for every object you want to add. It's be nice if you could just import objects into the sim directly, and then place it, without having to create the project files.
EDIT 2: Also I tried to reply to this yesterday before my account was verified, so I had the tab still open. Going back to the thread list, I can see someone else has now made a full tutorial on importing models, so hopefully that helps you out.