According to Wikipedia, they are both correct.
http://en.wikipedia.org/wiki/Arrow
Quote:
Nocks
The nock is a notch in the rearmost end of the arrow. It serves to keep the arrow in place on the string as the bow is being drawn. Nocks may be simple slots cut in the back of the arrow, or separate pieces made from wood, plastic, or horn that are then attached to the end of the arrow.[16] Modern nocks, and traditional Turkish nocks, are often constructed so as to curve around the string or even pinch it slightly, so that the arrow is unlikely to slip off.[17] In English it is common to say "nock an arrow" or "notch an arrow," when one readies a shot.
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So, when notching or nocking an arrow, you put the notch of the nock onto the string.
(That last sentence really feels made up, although it seems to be quite correct
)