How do you get Ho-oh on tin tower?

you have to get the Rainbow Wing from the Radio tower director in goldenrod city (Gold) after you beat Team Rocket there or from the man in Pewter City (Silver). After you have that, go to Ecruteak City and enter the Tin Tower. Travel up the Tin tower and on the Top Floor you will find Ho-Oh (LV. 40 in gold; LV 70 in Silver)