Try downloading the Sun Java runtime, and pointing directly to it.
In other words, if it installs to:
/usr/java/jdk1.5.0_09
you'd run JBidwatcher with:
/usr/java/jdk1.5.0_09/bin/java -Xms256m -Xmx768m -jar JBidwatcher-1.0.jar
Make sure you're using Sun's stable Java, not libgcj, OpenJDK, or IcedTea.