If you mean Model Railroader magazine, read the instructions on Page 83 of the July 2012 or any other issue at the beginning of the Classified Advertising section.
Schedule of Events Rate: $35 per issue (45 word maximum). And the following text outlines in detail what must be provided concerning the event.
In addition to the physical magazine, you can post an event to this web site by going up to the very top of this page, clicking on “Resources” and then on “Coming Events.” I believe that online listing is free.