The javac binary (and probably other java binaries) is/are not in your user's $PATH environment variable. There are several ways you can address this:
Add /usr/lib/jvm/java-6-open-jdk/bin to your user's $PATH environment variable. You can do this by adding a line similar to the following in your user's .bash_profile:
export PATH=${PATH}:/usr/lib/jvm/java-6-open-jdk/bin
You'll have to restart your terminal session for it to take effect.
Create symbolic links to the java binaries from some directory that's already part of your path (such as /usr/bin)
sudo ln -s /usr/lib/jvm/java-6-open-jdk/bin/java /usr/bin/
sudo ln -s /usr/lib/jvm/java-6-open-jdk/bin/javac /usr/bin/
BTW: There are several other java executables in /usr/lib/jvm/java-6-open-jdk/bin. I've shown the symlink commands for java and javac above. You should run similar command for any other executables you may want to use.
Use the fully qualified path directly on the command line:
$ /usr/lib/jvm/java-6-open-jdk/bin/javac
Update:
Apparently, there is an elegant, but Ubuntu-specific solution to this problem. When on an Ubuntu system, use update-java-alternatives.
