before adding RAC database as target, you have to add the hosts that hosting your database.
follow the following to add the hosts:
go to setup--> add targets --> add targets manually, then choose add hosts targets and click add host
click on add and provide the name for the host (this name must be resovable from the OEM server) and the O.S and click next.
then provide information on the host(where to install the agent, the owner,....)
then click next and finish to automate the depolyment of the agent
You can add target using command line also using emcli.
Below is the Eg.
For more details please refer this doc http://docs.oracle.com/cd/E24628_01/em.121/e17786/cli_verb_ref.htm#CACHFHCA