To do most things with another member, you do it from *their* profile. This includes adding as friends, leaving a reference, sending a couchrequest, sending a message, etc. Simply click on the other member's profile, find the appropriate link on their profile, and follow any instructions that follow.
If you would like to send a friend request to a member, click the "More" button to find "Add Friend" in the menu. Clicking "Add Friend" will send this member a friend request, which they will have to Accept in order for them to be added as a friend.
You can see all of your Couchsurfing Friends in the Friends Tab of your profile:
If you are wondering how to find your friend's profile so that you can add them as a friend, you can search for members using the Member Search feature from the navigation bar.
If your account is connected to Facebook, you will be able to see your Facebook friends that have Couchsurfing account from your Dashboard, or the Friends tab of your profile.