The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
You can try doing what it says. Open the start bar and search for “environment variables”, it should bring up a windows settings page where you can add one called EXE4J_JAVA_HOME with a file path to where ever you installed Java, or maybe even the Java directory that comes with the client.
