It’s always interesting to learn what a rich, acquisitive institution like the Museum of Modern Art has added to its collection. The latest report is the exhibition “Transmissions: Art in Eastern Europe and Latin America, 1960-1980,” which opens on September 5 and will present some 300 artworks in numerous mediums, many on view for the first time. Nearly all date from around 1968, a period when art radically diversified, sometimes dematerializing altogether.