Thank you for your quick and helpful response, Colmines92!
Did you use a special program for that? Could you also try it with another game because "The Earth Adventure" is actually the sequel to WAS IST WAS: Lost Treasures? In "Lost Treasures" you can also activate extras if you open the in-game...
In the game "WAS IST WAS The Earth Adventure TESSLOFF VERLAG" you can activate extras in the game if you open the in-game pc and enter a numeric code. Where can I find those codes when I look in the rom source code?