Your LolKing address is an optional field. It is used because LolKing offers very nice statistics about your Summoner Profile in a very easy to read format. This can be used by potential teams to see what your stats are.
To get your LolKing address go to www.lolking.net and enter your summoner name in the search box. You will then need to pick the correct result (look at the server). You can then get the URL from your browser bar.