Under RDS

To connect to your rds you'll need to have a security group as well a VPC setup.  

• Click on RDS > select Db > Connectivity and security
• Click the VPC link
• There is a section called IPv4 CIDR.  (This is what you need!)

Under EC2

• Click Ec2 > select instance > Security Groups
• Inbound > edit Custom TCP Rule > Port > Source = VPC Ipv4 CIDR