Oracle hints are supported from DbVisualizer 8.0. The following is applicable for older versions.
Block comments identified by /* ... */ are automatically removed by DbVisualizer when executing a statement in the SQL Commander. Since Oracle hints are identified by /*+ ... */ these are removed as well. To workaround this open Tool Properties and select the SQL Editor/Comments category. In Block Comment Begin Identifier, add a trailing whitespace after "/*" so the result will be "/* ".