^ upload merge folder to your MyBB files folder (eg. forum/merge) then run merge system by visiting forumURL/merge/
thanks
You don't have permission to access /forums/merge/ on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
^ check
permissions of folders & files. folders can have
CHMOD 0755 and php files 0644
if
mod_security is enabled then you may have to ask your web host to disable it or try using
.htaccess to disable it