Have you added a contrib non-free source to your apt source list? If not, use your favorite editor and add one, after it's added update your repository list,
apt-get update < must be root
Here is my non-free entry
Code:
deb http://ftp.us.debian.org/debian/ lenny main contrib non-free
deb-src http://ftp.us.debian.org/debian/ lenny main contrib non-free
Now open your synaptic package manager and do a search for
sun-java5-jre or version 6 sun-java6-jre and install it.
Edit: My source is for 32bit.