install Avahi Zerconf browser from Ubuntu Software Centre (or avahi-discover from Synaptic package manager) 

make sure the firewall isn't going to cause problems
$ sudo service iptables stop

run Applications->system tools->Avahi Zeroconf browser

find the _riousbprint._tcp printer

click on it and note the IP address of the Aiport Express

System->Administration->printing

New Printer->AppSocket/HPJetDirect

type the IP address of the Airport Express as the Host address

Select printer type and model and give it a name.

re-enable the firewall
$ sudo service iptables start

Job Done!

references