I take it you did cross pollinate since tropical hibiscus aren't self-seeding.
The flower will fall off and the base that's left will, over the course of several weeks, swell up, turn brown and then open up. When it turns brown, it's a good idea to cover this seed pod with something (mesh, pantyhose) so that when it does open up, you won't loose the seeds.
good luck! It's a cool process to watch...and even more cool when you actually see the seedlings and know you did it all.
Barb

Bringing this thread back up because I amazingly found a seed pod on my tropical hibiscus, an "Edward LaPlante", just last week. After 10 years, it has never done that and it hasn't bloomed this winter, although it still had blooms on it when I brought it inside in October.. I also assumed it was sterile. Well guess what? ;-)

I keep mine out on the balcony during summer so since I noticed carpenter bees lounging on it, I'm thinking they found someone else's unknown and pollinated mine or mine sported and self-pollinated... This is just a guess because not too many people in my hi-rise have plants out on their balcony (aside from typical windowbox annuals and some house plants) let alone a hibiscus out on their balcony... and certainly no one up on the 18th floor where I am.

The pod had 3 seeds which I planted this past Monday and I checked today and the things had the nerve to have sprouted. Will see how this all goes.