pgAdmin on a fresh OS X Server installation
Postgresql, just as most databases, can be accessed locally (on the local machine) and on TCP/IP, from remote machines. There is a common phenomenon that a database is configured by default for local connections.

The local connections can happen


on unix sockets, these are very like TCP ports, but they have a filesystem path instead of a port number. In case of PostgreSQL, the commons settings are 5432 for the TCP port and /var/run/postgresql/.s.PGSQL.5432.
On TCP ports, which is by default 127.0.0.1:5432, and is often by default turned off.  To turn it on, specify port = '5432' in postgresql.conf.


Most clients - php postgresql module, or the command line psql command - connecting the local server on unix sockets by default.

pgAdmin is a special thing, it wants to connect only to TCP ports, even if it runs on a localhost. It doesn't mean anything on your system were bad, this tool was simply so written, that it can use only TCP even for localhost administering.

You had to enable the TCP port listening in postgresql config file pg_hba.conf. The following line will be probably okay:

host all all 127.0.0.0/8 trust


The official documentation for this pg_hba.conf can you find here, and a short tutorial to enable the localhost networking can you find here.

If it don't work or you want better answer, it could help a lot if you extended your question with your actual pg_hba.conf.

Good luck!