^This heater correct?
Correct me if I'm wrong, but I do believe that is a Present heater, meaning that there is no thermostat and its not adjustable. Unfortunately these heaters are often unreliable and either don't work at all or under-heat. To be honest, I'd return it if I were you and instead look for a 50 watt thats adjustable with a thermostat. Hydor, Elite, and Aqueon are a few good brands that come to mind.
As far as heater placement, next to the filter intake either vertical or somewhat tilted(my personal preference, but to each their own)should do the trick.
I personally don't find any heater difficult to hide if you have enough cover in your tank, though at the same time I don't find heaters to be unsightly....they're just part of having an aquarium like anything else. xD
You should always have your heater plugged in. Some people who live in a warmer area with stably-warm summers can get away with unplugging their heaters in the summer, but its usually a good idea to keep your heater plugged in 24-7 year round, just like a filter, to keep your water stable.