Okay, asumming you already download ymessenger_1.0.4_1_i386.deb.

Open kosole and type this command:
su <enter> **it will ask for root password, enter that password
apt-get update <enter> ** you don't need to run this if you already done it before.
apt-get install libssl0.9.6 <enter> **this will install libssl0.96 from net

Then change to the directory where you save ymessenger_1.0.4_1_i386.deb.
ex:
cd /home/yourname/download <enter>
dpkg -i ymessenger_1.0.4_1_i386.deb <enter>
ymessenger <enter> **this will run ymessenger and will ask you to configure your yahoo messenger.

You can also install .deb package using gui (kpackage) if it is installed in
your system.