do a:

apt-cache search kopete

to see if it is in your package lists.

as well as trying:
apt-get -s install kopete/unstable

you might want to try:
apt-get -s -t unstable install kopete ( and maybe -t stable)

IIRC apt-get -s install kopete/unstable and apt-get -s -t unstable install kopete function slightly different. You will have to have a repository URI in your sources list that has the kde packages you want, you might want to look at www.apt-get.org for unofficial package repositories and their sources.list URI's.

HTH

rock