Head north from Iccirus City. But your tag says "heartgold" and if you're wondering that:
You have to get the 4th gym badge.
Once you save the Radio Tower, from Team Rocket, you'll get a Rainbow Wing.
Show the wing to the sage, and he'll let you go.
If you have the Clear Bell, Ho-oh will appear at the top.
And for the record, it's the "Bell Tower."