To add a friend you have to first send that person a 'friend request'. You will see it in the button that says 'Add as friend'. You have to write a message telling that person about yourself, your goals in the language that you are studying, and how you can help the other person learn your language. Send requests to people who are learning your language, and clearly show how you can help them, otherwise they might just ignore your friend request. Don't give up, keep sending requests until you find someone.

I think some people set their profiles so they can't be added.

because a general ID only add 3 person one day.