If short, from now to install libmysqlclient15off, go to http://archive.debian.org/debian/poo...ysql-dfsg-5.0/
Download libmysqlclient15off_5.0.51a-24+lenny5_{YourArch}.deb and install it.

Code:
wget http://archive.debian.org/debian/pool/main/m/mysql-dfsg-5.0/libmysqlclient15off_5.0.51a-24+lenny5_amd64.deb
sudo dpkg -i ./libmysqlclient15off_5.0.51a-24+lenny5_amd64.deb