I have put together a video demonstrating how to enable PostgreSQL in Mountain Lion Server 10.8, and then how to connect to it using an application based management tool (such as pgAdmin3).
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Update – 01/02/13
As mentioned by Jeff Johnson in the YouTube comments, it may be better to make the listen_addresses change via Terminal rather than editing the plist. Here’s how:
In Terminal, type:
sudo serveradmin settings postgres:listen_addresses = "127.0.0.1"
sudo serveradmin stop postgres
sudo serveradmin start postgres
This should stop any issues people are having with Profile Manager and Wiki afterwards.