I used RHEL 4 with Oracle 11G.
Hopefully the commands below will help: (The following commands assume you have $ORACLE_HOME environment variable set)
To view the status of EM:
$ORACLE_HOME/bin/emctl status agent
The last line of the output should be:
Agent is Running and Ready
If not, you need to start the DM console:
$ORACLE_HOME/bin/emctl start dbconsole
To grab the URL required to connect:
$ORACLE_HOME/bin/emctl status dbconsole
If you cannot connect configure your firewall to permit port 1158.