When composing a new PM once you type in the name you have to click the "Add" button. I've been meaning to adjust the layout and make it a little easier to understand. Once you do that the persons name will appear in the "To:" field
This picture is what it looks like after hitting the "Add" button.
If you just type a name into the text field and hit the submit button without hitting the "Add" button it will bring you back to the compose panel but it will now have a preview and then members name added . Hitting submit at this point will send the PM.
The other possibility is that the member has checked off the ability for people to send them PM's. I'm not sure what kind of error that throws.