You can add your Steam friends by clicking on the Add Friends button
in the friend info window or the link below.
[Add friend link]
If you can not find your ID in the Friend Finder, go to your friend's profile page.
There's even able to add a friend.
If it is not possible to add a friend on your profile page,
your account may be in the "Profile private" state.
You can not add profile friends while the profile is private.