I just installed ubuntu server 12.04. The server detects the modem, but I can't edit mobile broadband settings (to input username and password) in network connections
If you are asking how to do it-
As pointed out by @Eliah kagan,my answer points to a graphical way that may not be available in Server edition.And as suggested by him,I keep the answer intact with an addition of a possible useful link.
This link may help for the server edition- http://www.sourceguru.net/setting-up-networking-on-an-ubuntu-server/
While for a graphical way-