I don't seem to be able to do this..........would anybody know what may be wrong?

I assume you're trying to contact a member of MM.  They are the only ones you can contact directly through MM.  the simplest way is to just go to that member's profile.  In the upper right hand corner of the profile is a group os symbols, one of which is an envelope.  Click on that, and a new page pops up. Type your message and at the bottom of the page click on "Send". and  Bingo! your message is on its way.