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