Host – The Cloud Control database plug-in must be deployed on the Agent monitoring the selected host.

Deploying the Cloud Control database Plug-in on the agent in 12cR1.
Step 1: Go to Setup -> Extensibility -> Plug-in

Step 2: Management Agen

Step 3: Check version Agent -> Continue

Step 4: Select Host Agent

Step 5: Prerequisites Check

Step 6: Deploy the Agent on the host

Step 7: Validation

Step 8: End of the Plug-in deployment

Conclusion:
Within the Oracle Cloud Control 12c database Plug-in.