Now start the server and check wether he is loading the driver. The console should show something like: INFO [org.jboss.as.connector.subsystems.datasources] JBAS010404: Deploying non-JDBC-compliant driver class org.postgresql.Driver (version 8.4). If it nothing showing about your Postgres Driver, check the spelling and the coding (e.g. the quotation marks can be in different code and dont work, can happen if you just copy and paste). Fix this before going on.
Go to your standalone.xml and set between <datasources> following code (set your connection-url, usename and password, jndi-name can also be java:/postgresDS, you can do this step through admin console of the server as well):