If you changed to static (fixed) ip you need to set you DNS server too. It is the ip address of your router for most people. (e.g 192.168.1.1)
/system/network then on right the DNS Server tab int he OMV web-gui.
This probably why your system is not finding packages
PS- Stick with apt-get (APT) for package management. Aptitude is not installed. OMV uses APT for package management and it is best to stick with that.
Manage firewall with webmin: http://forums.openmediavault.org/viewtopic.php?f=3&t=1072
How to install OMV Extras in OMV .5(Sardaukar): http://omv-extras.org/simple/index.php?id=how-to-install-omv-extras-plugin