In Mountain Lion, the new Messages app replaces iChat.
If you’re the kind of person like me who religiously keeps your email, chat logs, etc, but you have chosen not to do a migration of your data when upgrading to Mountain Lion, you’re going to be faced with the problem of how to import your old iChat logs into Messages.
It turns out that Messages stores logs in:
~/Library/Messages/Archive
They appear to be the same format as iChat, so if you move your old logs into this folder, Messages will find them.
Alternatively, you can make a symbolic link to a folder elsewhere and put your logs in there. For example, this will rename the old Archive as “Archive.old”, then make a link to a folder called “iChat” in your Dropbox folder.
> cd ~/Library/Messages
> mv Archive Archive.old
> mkdir -p ../../Dropbox/iChat
> ln -s ../../Dropbox/iChat Archive
This just leaves the Archive.old folder lying around, but you could obviously move its contents into your new iChat folder then delete Archive.old.