Go to the profile of the person you want to message.
Click in the "Mail" tab at the top of their profile (not yours) between the "Profile" and "Photos" tabs.
It will take you to the mail page where you can contact them.
When using mobile, go to their profile. You will see a tab that shows " Profile " and has 3 strikes at the right of it as well. If you click that, options will appear for that profile, including the mail.
If you want to view your own messages on mobile, you will always see a letter shaped symbol at the upper right corner of the web page.