These should be the green blobs which drop when you kill enemies, when you press E while near them it opens up bodypart menu with the bodypart in the stash. After that just drag and drop it into the correct slot and boom, stats increase.
Oh, ok. I didn't see any of those when i first played it, but This time i found some. i got stuck for a minute on the Book screen, but after restarting it worked like a dream.
