two issues here. Your WSDL should not use rpc/encoded this is disallowed by the WS-I Basic Profile (BP).
The wscomile tool is part of the Sun JWSDP, and therefore will generate a jaxrpc-mapping.xml file that works for the classes that wscompile generates. If you have complex types like tns:SessionCategories you should check the generated mapping and modify it accordingly (i.e. map the types you actually use)
An alternative approach is to use an external ws4ee-deployment.xml file for even more control over mapping and serializers, etc. See the wiki.