The following matrix shows what Java version is necessary to run a specific Weka version. It also specifies when a changeover happened, listing date and Subversion revision (the number prefixed with r).
Note that Java 5.0 and later in combination with Linux/Gnome has problems with the default Look'n'Feel. A workaround for the problem was introduced with version 3.4.5/3.5.0. For more information refer to the Troubleshooting Wiki article. From Weka 3.6.5/3.7.4 Mac OS X users will need Java for Mac OS X 10.6 Update 3 (java 1.6.0_22) or later.