Did you install Debian after booting from
this installation image? The Debian image for USB sticks doesn't include much software and is intended for booting the installer, while downloading the rest of the system from an Internet mirror. What you should do, if you want to be able to install Debian software without connecting to the Internet, is to burn (or buy) a set of CDs or DVDs and run "
apt-cdrom add" for each one of them, removing (or commenting out) other entries from your
sources.list file.