I'll be monitoring this thread for a while until I get the forums up on the website.
Take the URL of Sauron's profile on the website as an example:
The 19667000 is for Riot to identify the player. To get that number, go to lolking.net and search for your player. I searched for Sauron, and the URL of his profile on lolking is:
So if you want to look up your elo on lolmatches, just search up your profile on lolking.net and look for the number. Copy that number and paste it in this format: http://lolmatches.com/player/na/(your number)/
EDIT: Discworld ninja'd