select i from Item i, Category c1, Category c2
  where c1.kind = 'apple' and c2.kind = 'orange'
  and c1 member of i.categories and c2 member of i.categories

//the list that will contain all the components from your AND statement
List<ICriterion> ANDExpresionComponents=new List<ICriterion>();

//add all your components here
ANDExpresionComponents.Add(Expression.Eq("Categories", "apples"));
ANDExpresionComponents.Add(Expression.Eq("Categories", "oranges"));

........................................................................................................

//get the NHibernate Session
ISession session=NHibernate.Instance;

//this criteria will return the records that match all the added criterias
ICriteria criteria=session.CreateCriteria(typeof(ITEM));

if(ANDExpresionComponents.Count>0)
            {
                ICriterion crit = ANDExpresionComponents[0];

                for (int i = 1; i < ANDExpresionComponents.Count; i++)
                {
                    crit = Expression.And(crit, ANDExpresionComponents[i]);
                }

                criteria.Add(crit);
            }

IList<ITEM> result=criteria.List<ITEM>();

select c.itemId from Category c where c.kind = 'apples' or c.kind = 'oranges' group by c.itemId having count(c.itemId) >= 2