Under the tab "My Messages" underneath the nice Rootschat picture you'll see "send a message". That brings up a mail area. His title is simply seaweed (no capitals). Then just use as normal email. To save explaining it all, you could ask him to look at your thread (Training Ship Cornwall) on the Beginners board and ask if he can help with pointers towards records for your man prior to the seaman's record you have already found.
maxD