Enterprise manager can be started as follows:-

$cd $ORACLE_HOME/bin

$ emctl start dbconsole
TZ set to US/Mountain
Oracle Enterprise Manager 10g Database Control Release 10.1.0.2.0
Copyright (c) 1996, 2004 Oracle Corporation. All rights reserved.

http://localhost.localdomain:5500/em/console/aboutApplication

Starting Oracle Enterprise Manager 10g Database Control ……………………… started.

Various options for emctl are:- emctl start| stop| status dbconsole