I added the three "cloud addresses" they provided in my inbound rules for RDS security group, but no dice. I can connect to this db instance from the Metabase MAC app and from pgAdmin.
Have you also added the IPs of your computer and pgAdmin to the inbound rules? I'm asking, since it seems rare to add your home IP, usually a VPN or SSH tunnel is used instead.
I'm guessing these are the addresses you've added: IP to whitelist for Metabase Cloud instances
What do you mean about "no dice" - do you get a timeout after 5 seconds, or another type of error? Check Admin > Troubleshooting > Logs.
nevermind i fixed it! i didn't select the correct connection type to allow in aws - i had htts instead of postgresql in the unbound rule. duh!