is it a separate sidegame included as an extra with creatures 3? it won't let me register an account for it.

Creatures Docking Station can be used a standalone, or in conjunction with Creatures 3. The online registration process has been down for years, since the server is no longer maintained by the original company. However, you are able to install a third-party addition that allows you to play offline. This is included with the metaroom C1toDS, or you can also download the DS Offline Option from here. Hope this helps you out! Best of luck!