If you upgraded from an earlier version of StreamedMP to v1.6 its possible you had a BasicHome menu item pointing to MyTrailers. This plugin is not supported in MediaPortal 1.2 Beta and is no longer being maintained so we dont include that plugin anymore.
As a replacement we now map our Trailers menu item to OnlineVideos->iTunes site. So if you are a clean install all you need to ensure is that you have OnlineVideos installed, if you upgraded you will most likely also need to open the BasicHome editor and delete your existing trailers item (which pointed to mytrailers) and add it again which includes the new information.
If you are refering to the 'Trailers' button in MovingPictures/MyVideos then you need OnlineVideos and the IMDB site enabled.