sorry if I'm asking something obvious, but your mesh editor is a bit confusing
anyways, how can I set different textures/materials on different faces of single object (cube, for example)

Hi,

you need to have model exported into multiple parts (one file but made from multiple sub groups)
then in the mesh Editor you can select different mesh part with - = keys (its the Menu\Edit)
then when applying a material it will be set onto currently selected mesh part only
check the Mesh Editor FAQ for more info (english.chm\tools\mesh editor)

got it, thanks a bunch. (though it would be more convenient just to click the face and then add material)

thanks, I'll keep that in mind