You probably don't have the "testing" in your /etc/apt/sources.list
For example, my config looks like this:
# I don't want any stable (woody) packages
#deb
http://ftp.fi.debian.org/debian/ stable main contrib non-free
# I do want testing (sarge) packages
deb
http://ftp.fi.debian.org/debian/ testing main contrib non-free
# I don't want unstable (sid) either
deb
http://ftp.fi.debian.org/debian/ unstable main contrib non-free
After updating the file, do a "apt-get update". I also suggest installing aptitude, it it a very powerful frontend for apt-get.