aay made small mistake: the command should be:
Code:
apt-get -t unstable install kopete
(note the "install", thats the missing command apt-get complained about
you could've found that out easy by reading the man about apt-get... THATS how you learn, not by asking us...

how to get info about a command?
Code:
man (command)
so
Code:
man apt-get
gives manual of apt-get. you can use this for every command!

good luck...