The configuration of the server is incomplete. Try executing those:
<pre>
Defaults write NSGlobalDomain LSAdaptor PostgreSQL
Defaults write NSGlobalDomain LSModelName OpenGroupware.org_PostgreSQL
</pre>
Note that this should not happen anymore with OGo 1.0a releases. Always ensure
that your database server is running using:
<pre>psql -h localhost OGo OGo</pre>
(replace the first OGo with the database containing the OGo database schema
 and the second OGo with the user you use to connect PostgreSQL)