You can find the HTTP port allocated to the Oracle HTTP Server by using the opmnctl utility.
$ORACLE_HOME/opmn/bin/opmnctl status -l
This should spit out a table showing all the managed processes and also the ports they are currently asssigned.
Look for the HTTP_Server entry and the corresponding HTTP port it is using, then use that as the server:port entry to access EM.
Typically on Windows we default to port 80 and on the *nixes we default to 7777. If those ports are not available at install time then the installer will increment the value sequentially until if finds a free port.