Hi ,
You can add your Listener for Monitoring. by following steps..

On EM go to Agent Home page (Agent target will be like hostnameort)
for the node where Listener is running.
from Add combo box select Listener and click go.
It will ask you few details like
1) what Name you want to give to target.
2)listener.ora locatio
3) port
and few more.

Just go through next next and Monitor your Listener