In the top row, on the right-hand side, next to your username, there should be an icon of an envelope. If you click it, it pops up and you can click on "Send private message". Then you can add the user you want to send it to and the text. This should be reachable from any part of the forum.
You can also hover of a username and it will pop up some information about the user, as well as a button to "Send direct message", where the user is preselected as recipient.
Users may have disabled someone being able to send them private messages.