Powerhead Position: I assume you are using a powerhead with tubing going directly into the intake. Position this 1/4 from the top of the tank with the output attachment (if you have one) pointed downward.
Hornwort Planting: Hornwort is naturally a free floating plants. However you can use lead weights attached loosely at the base of a bunch to hold them down. You can also plant them into the gravel, but as you'll find out hornwort breaks pretty easily, and this method isn't as effective sometimes.
-John N.