Need help for writing this query in LLBL expression without Linq

Select * from Customer where CusDate= (Select CusDate from Customer where CusID = @ID)

Using LLBL 2.6 Adapter mode SQL 2005

You can use an ScalarQueryExpression :

ScalarQueryExpression specificDate= new ScalarQueryExpression(  
    CustomerFields.CusDate, CustomerFields.CustomerId == someCustomerId);

IRelationPredicateBucket filter = new RelationPredicateBucket();
filter.PredicateExpression.Add(CustomerFields.CusDate == specificDate);

// fetch
var customers = new EntityCollection<CustomerEntity>();
using (DataAccessAdapter adapter = new DataAccessAdapter())
{
    adapter.FetchEntityCollection(customers, filter, null);
}