I received a message which said that I do not have permission to access the post on the horse abuse rally in New York. Why and how to I change that status as I am not a new user and do not think I should have to send in another activation message? Or do I?
I could access the post to read it, but received the same message as you when I hit reply. In reading the rules, only premium members can submit a thread, and I assume that only that member can add to the thread, as in the lost horse forum.
Yup, that’s correct. Both the Event Announcements and Missing Horses forums only allow posts by the user who started the thread. 
