The Enterprise manager console is very different in Oracle 10g from its previous versions. ANd what I understand you cannot use it to monitor databases that are not of version 10g.
The product is much sleeker and has a lot more power than its previous versions but is not backward compatible. It is now called 'EM Database Console'. You must have the dbconsole process running to access EM Database Console from a client browser. This process is automatically started after you install 10g on your server.You can access the console by opening your Web browser and entering the following URL:
http://hostnameortnumber/em
Hostname is the name or address of your computer.Portnumber is the EM Database Control HTTP port number that is specified during installation. The default port is 5500. You can find the value for your system in the '$ORACLE_HOME/install/portlist.ini' file.

Hope this helps.

-Chintz