To install Sun/Oracle java 6 on Ubuntu Lucid do the following in a Terminal window:

sudo apt-get install sun-java6-jre sun-java6-fonts sun-java6-bin sun-java6-plugin

If this says: E: Package sun-java6-jre has no installation candidate then do:

sudo bash -c " echo 'deb http://archive.canonical.com/ubuntu lucid partner' >>/etc/apt/sources.list "
sudo apt-get update

And try again.

When you have installed the packages you should do:

sudo update-java-alternatives -v -s java-6-sun