Update: as of November 2015, IC has been made available on Steam and GoG by Nordic Games, including restored Impossible Creatures Online servers. To play online, all you need to do is select "IC Online" from the multiplayer menu and create a new account.
Firstly, since the abandonment of Impossible Creatures Hive and death of the Creature Chaos mod, the IC community has moved to a new forum: Ic-tellurian.proboards.com (Note: Though posts here will certainly not be ignored, it is recommended to use the forums for any troubleshooting regarding Tellurian as replies will likely come much quicker.)
As the URL suggests, Tellurian has now become the standard mod for Impossible Creatures multiplayer games and all new members are highly recommended to upgrade to version 2.02 if they would like to partake in IC multiplayer within the community.
As for multiplayer itself, gone are the days of playing via LogMeIn Hamachi. Though Hamachi was a decent service and played a huge part in keeping the game alive for so long, we have found that games played via direct IP (Internet TCP) lag far less and in general are much better experiences. However, this means that less people are able to host games successfully and, as such, Hamachi is still used as a last resort.
Since we don't have Hamachi's chat interface to rely on anymore, the community has turned to using Skype as its primary means of communication and game organisation. To take part, please add "puglinic" (sans quotes) as a contact on Skype so that you may be added to the relevant group(s).
TL;DR: New forum here: Ic-tellurian.proboards.com
Add puglinic on Skype if you'd like to play online.
We sincerely hope that you will join us.
~The Tellurian Development Team