If there is no Java 6, then unfortunately you won't be able to use LW 5. Unless you upgrade your OSX to Snow Leopard 10.6
If Java 6 is not listed in Java preferences, it means your computer is not compatible with the 64-bit Leopard version of it. Snow Leopard has a 32 bit version of Java 6. Blame mac for that.

Your options if you don't upgrade to Snow Leopard are to use an earlier version of LW Pro. Go to your LW Pro download page at their site & see if any earlier versions are shown. If not, let me know.