Reminder if you want private messages on this forum....

You could look in their profile - see if they have posted anything, and then write to them in that thread.
And yes its not obvious at all - I made exactly the same mistake trying to pm someone. it doesn't warn you that you won't be able to get replies.

If the 'mods' are reading this - if its possible to have private messages turned on by default instead of off - that would be better.