It will take about a full day for a heater to bring a tank of room-temp water up to the temperature it's set at, especially since 50W is on the small side for a 10g tank. A heater is only on for a few minutes at a time (so it doesn't get too hot and burn the fish, or itself) and then is off for a period of time, so it takes a while to see major changes.
Since you already have fish in the tank, just let the heater do its thing and check on it every hour or so to make sure it's not getting too hot and needs to be adjusted down.
The placement of the thermometer doesn't really matter as long as the bulb that measures the temperature is fully inside the water, and you can read it easily. Though I'd suggest not putting it within about two inches of the heater so it doesn't read too high of a temperature from the heater working. I like having mine on the front or side walls of my tanks.
And yes, ideally you should never plug in a heater outside of water. A few seconds just to check that it's running is usually alright, but only if it was cool to start with and was unplugged again right after. Never let a heater that was just in a warm tank run outside of water, or run for longer than it takes to see if it's working, because that (and pouring cold water onto a hot heater) is when the biggest risk of shattering occurs.
