I had this problem with my Mac after upgrading to Snow Leopard. Turns out they removed JVMs older than 1.6. I had to find some instructions on how to put the 1.5 JVM back in so that I could select it in the Java Preferences app (under Utilities). I could not determine how to give the app the path to a different JVM easily, but this worked.
Had the same problems with all mc4j versions (old ones too). Eventually switched to JDK 5 and the NPEs stopped. You can change the Java runtime that the application starts with in the 'MC4J Console 1.2b9.lax' file.
