Where is it?

It depends upon where you bought the game. Each site handles the protection and registration process for the games that they sell. If you bought it directly from LDW, you can get help by filling out the support form. If you bought it from another site, their customer support should be able to help you.

Doesn't this question get asked a lot? It should be in a FAQ of some sort.

There is a post near the top of the Fish Tycoon forum that explains registration issues for Fish Tycoon for Windows. We should probably expand that to include the recently-released Fish Tycoon for Mac.

Yeah the people who support Mac really don't send out answers fast enough. My fish are all probably dead, but I can't get on b/c I deleted my activation code b/c I didn't realize they would ask me again for it! I'm mad b/c I paid 20 bucks and I can't get back on!
They don't even have my email on the database, even though I get emails from BIGFISH GAMES.

I can understand your frustration. Each site that sells the game packages it with their own protection software, so registration codes can come only from the site where you purchased the game.