Let the cursor point to the person's name you want to send the PM to and a panel will drop. At the bottom of the panel, you'll see "Start Conversation", click on it.
I tried clicking on where it shows start conversations. After I wrote a message then it shows need title and message.
For me the old way was better. Are the messages private or for anyone that goes to their username? Because I see two boxes. One says start conversation with other people and one says lock.
When you start a conversation, only yourself and the people you included can see the conversation thread. If you click on "lock", you have closed that conversation so that neither you, or the others you included, can add any more messages to that conversation.
It's like in the old PM system, you can include more than one recipient. Once you've written the message and selected the recipients, you can click on the submit button at the bottom. They'll receive a notification and when they get on the forum, the little envelope at the top of the page, will show they have a message for them to read and, optionally, respond to it.
Just to show you, I'll send you a PM (conversation).