Pond not looking great

I can't remember about the hornwort, but I think mine came with the weights as described by @pete. Most of the pond suppliers's sites will give you the info though. If you got yours online, just check that.
Many pond plants die back and drop and then resurface when they come into growth when the time is right for them, so just be aware of that too.
I forgot to mention that a good plant for your pond could be water hawthorn. It covers the surface, and is good for shadier spots. Nice white flowers, and seeds around once established.

I find watercress very good, and you just chuck it in. It'll root and move around, so if you have a spot where it's quite shallow, that's ideal to start off with. You do need to have piecs with stems so that it takes. I sometimes push pieces that are growing, but floating around, under a small rock or similar, to keep them in various places. It might be a bit late for it to do much now, but you can try it again next year. I don't think it's particularly easy to grow, because it does best in moving water, but you could always try growing some on in a container and see. I know I tried it once, but without much success because of that.