Found the answer myself.
we should have a user as Administrator, who creates the task by deafult.
So the complete process for the configuration of JBPM5 with LDAP
1) changed web.xml in jbpm-human-task-war.war in jboss7 deployments folder.
modified user.group.callback.class value to LDAPUserGroupCallbackImpl from DefaultUserGroupCallbackImpl
<init-param>
<param-name>user.group.callback.class</param-name>
<param-value>org.jbpm.task.service.LDAPUserGroupCallbackImpl</param-value>
</init-param>
2) updated jbpm.usergroup.callback.properties with requried properties
for e.g. ldap.user.ctx, ldap.role.ctx,ldap.user.roles.ctx, etc and kept in the war file, updated jbpm.usergroup.callback.properties the same in
jbpm-human-task-core-5.3.0.Final.jar also in the lib of the above mentioned war, originally that file was empty.
3) Create a user as Adminstrator in your LDAP server.
4) All the users in the bpmn as the ActorID must be there in LDAP directory.
5) Run your program in my case its just the sample program -> Evaluation.bpmn/ ProcessTest.java