Cisco Firewall :: ASA 5510 - How To Assign Multiple Public IP Addresses

I'm currently replacing my ASA 5505 with a 5510. I have a range of public IP addresses, one has been assigned to the outside interface by the setup wizard (e.g. 123.123.123.124 ) and another I would like to NAT to an internal server (e.g 192.168.0.3 > 123.123.123.125). On my asa 5505 this seemed fairly straigh forward, i.e. create an incoming access rule that allowed SMTP to 123.123.123.125 and then create a static nat to translate 192.168.0.3 to 123.123.123.125. Since I've tried to do the same on the 5510 traffic is not passing through so I'm assuming that the use of additional public IP addresses is not handled in the same way as the 5505? I also see that by default on the 5505, 2 VLANs are created, one for the inside and one for the outside, where as this is not the case on the 5510. Is the problem that VLANs or sub-interfaces need to be created first?  I'm doing the config via ASDM.
 
Everything else seems to OK i.e. access to ASDM via 123.123.123.124, outbound PAT and the site-to-site VPN.