Ok, I found a working site with some of the Modelgradas kits.
Actually, is the original site of the designers of the kit ("Nog Studio")
which apparently was some sort of sister site to Modelgradas.
There you can find the download of Joseph the Robot from Machinarium,
complete with the simple instructions guide. And some other kits.
Link is:
NogsStudio - Modelgradas
The site is exactly as I remembered it years ago. So they have not produced more kits in the past years.