The key has to be activated on: http://www.thesims3.com/registeragame.html . Then you may download the game from Origin using the same login and password as on thesims3.com.

Play with your own island paradise!

New adventures await your Sims on the sunny shores and beneath the glistening water of an island paradise! From exploring the tropical isles to creating a five-star resort experience, your Sims can decide to embark on an unforgettable journey or make themselves a new home in this new utopia. Nautical Sims can even take the helm of fully customizable houseboats and set sail between islands! If your Sims would rather be in the water than on it, they can snorkel and scuba dive their way through the ocean. Who knows, they might uncover sunken treasure or befriend a mermaid! With innovative features never before seen in The Sims PC franchise and a world of new possibilities on land and in sea, your Sims will reap the benefits of their own island paradise.

Minimal requirements:
 

Processor: 2.4 GHz - Pentium 4

Graphics: 128 MB VRAM - NVIDIA GeForce 6 / ATI Radeon 9500

Memory: 1.5 GB RAM

Disk space: 7.1 GB HD

System: Windows XP SP2 / Vista SP1