You go to their profile and on the right side of the page, you'll see a big orange button that says "follow." To the left of that button you will see another button that says "message." When you click that, it will bring up another page and that's how you message them. Some people have different settings though. Some users only allow messages from users they are following so in that case, you wouldn't be able to message them unless they followed you first.
In addition to what @mistninja314 said, a lot of people also have their settings to block messages from people not on their friend list. I have mine set that way because I get tired of random guys messaging me for cyber sex or nude pics.
If they have set there profile to only receive messages from followers they follow, you have to follow them first and they have to accept it/and follow you back for that to be possible. Otherwise I'd they don't have that feature on, you should be able to message them. Getting level 2 would help aswell.
Just click on the profile you want to message and click message. It's right next to the block button. You can test on me if you'd like :) Or it could be blocked in your settings possibly. I also hear that you need to be at Xper 2 to message but I'm not entirely certain about that