Make sure the following are set correctly.

1) Is your OEM and the OMS belong to the same version. If not upgrade them to be of the same version. You can also download the OEM 2.1. from technet.oracle.com or make a request to oracle for OEM 2.2 if you have the product upgrade support. This is the primary thing you have to have.

2. As sreddy pointed out make sure your intelligent agent is up and running on the server side. You can check it through
"lsnrctl dbsnmp_status" if not start that "lsnrctl dbsnmp_start". If you have any problem in starting it pl. let us know, I'll send you some trouble shootings

3. Make sure that your prefered credentials for database is SYS or SYSTEM the username and the password and the SYSDBA

4. Make sure that the user name for the node to be the user name and password of the user who has the rights to run the required jobs on the host.

Hope this would help you.

Sam