Just copy & paste the following HTML-code to your page:
<iframe src="http://steamstats.steamtracker.com/iframe/?steam_id=STEAM_0:x:x" width="250" height="60" scrolling="no" marginheight="0" marginwidth="0" frameborder="0"></iframe>
Make sure you replace STEAM_0:x:x with the Steam ID of the user you want to be displayed.
New: instead of the old Steam ID STEAM_0:x:x, you can now also use the new 64-bit Steam ID format 7656xxxxxxxxxxxxx
Here a tool for converting between the different formats: http://steamidconverter.com/