If you are using an USB modem for the first time in Ubuntu, you may face issues in connecting it. Becasue, http://rcm.amazon.com/e/cm?t=tecare-20&o=1&p=8&l=bpl&asins=B003Y3BM0W&fc1=000000&IS2=1<1=_blank&m=amazon&lc1=0000FF&bc1=000000&bg1=FFFFFF&f=ifrUbuntu will recognize the USB as CD drive and hence connection will not happen. You need to install a deb package to make the USB modem identified by system. You can find this package at below link.
Install this and insert your USB. In Ubuntu 10.04, it will take some seconds to USB become ready. You can see connection disconnected message. Go to network manager and edit your connection settings viz internet provider, username, password, and dial up number.
Please post comments if you need any assistance.