'libmysqlclient15off' is an 'Ubuntu 8.04 Hardy' package.
Add 'hardy' to /etc/apt/sources.list :
Code:
deb http://76.73.4.58/ubuntu/ hardy-updates main
deb http://76.73.4.58/ubuntu/ hardy universe
deb http://security.ubuntu.com/ubuntu hardy-security main
deb http://security.ubuntu.com/ubuntu hardy-security universe
And then : sudo apt-get update && sudo apt-get install libmysqlclient15off
..