I've had difficulty finding some people's Steam profiles using the normal methods. This should help if your friend has an unlisted profile or a very common username (or if, for some reason, Steam just can't find the person you're looking for).
Here is a way to get the Steam ID number of players on your Steam Friends List, if you're having difficulty:
Right click on the name and click "View Steam Profile"
This will open the Steam browser. Right click on the background of the page and click "Copy Page URL"
Paste the URL. The Steam ID is the long string of numbers after "
http://steamcommunity.com/profiles/ ". Copy that number into the SteamID search field on
the Player Search page to find your friend.
If you get a customized URL (eg
http://steamcommunity.com/id/onkelsam ), you can use
the Steam ID converter.