Ubuntu 10.04 64bit: Arduino program doesn't run

When I try to run "arduino" nothing happens. When I try the same thing from a terminal I get the following error message:

Exception in thread "main" java.lang.NoClassDefFoundError: processing/app/Base Caused by: java.lang.ClassNotFoundException: processing.app.Base at java.net.URLClassLoader$1.run(URLClassLoader.java:217) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:205) at java.lang.ClassLoader.loadClass(ClassLoader.java:323) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294) at java.lang.ClassLoader.loadClass(ClassLoader.java:268) Could not find the main class: processing.app.Base. Program will exit.

Apparently a problem with Java. However, I installed Java exactly as explained in "Complete Numpties Guide to Arduino on Ubuntu 32 & 64 bit" on the Arduino website. I didn't see any error message during that installation.