The problem when you are using USB modem in Ubuntu, is, sometimes you will need wvdial but wvdial itself need to be installed first. Fortunately, Ubuntu included wvdial in its official ISO since long time ago. Further, it means wvdial is available in Ubuntu LiveCD but it is not when Ubuntu have been installed. In LiveCD, just command sudo apt-get install wvdial and you install it completely without internet. Yes, Linux Mint has same behavior too. It is probably a secret only few people know. To install it, just insert your Ubuntu CD (same version, same architecture) and command apt-cdrom it to be your additional repository.