Find the Member you want to Private Message by going to DATING -> Members
You will see something like this:
In the top right corner you can do a Search for the Member you wish to message by Username, or you can view the members list by "Last Active", "Newest Registered", or "Alphabetical",
You can go to DATING -> "Member Search" page and search by Location, Sexual Identity, Age Range, Username or keyword.
Once you find the member you wish to "Private Message", click on their avatar or username link to be taken to their profile, which will look something like this.
From there you can send them a Private Message, a Compliment or ask them to be your Friend.