First start the Blender and you come to this screen. (Object mode)
THE EXPORT PLUGIN IS IN BLENDER AS DEFAULT!There is the default cube move your mouse over the cube and press right mouse button to select it. After that you see "File" in top of the screen press it and it opens the list and down there you see "export"
after its pressed. new windows pops up (press the "Export sel"
and give it a name and save it now youve created a a model. now throw the Model file to universe sandbox/Media folder and open the game. Spawn planet (sun, moon, anything... after you spawned it go to the planets psychical properties and change the appearance "model" to the name what ever you gave the name to the model. i gave name "Test" for the model.
Congratulations!, your model is now in the game.