If you use this method 
 then you can form SQL WHERE like this:
        string where = "UserName NOT LIKE N'public'";
         // Do not select hidden users
        where = SqlHelper.AddWhereCondition(where, "((UserIsHidden IS NULL) OR (UserIsHidden=0))");
        ConnectionHelper.ExecuteQuery("somequeryname", null, where)
Don't forget  query must contain ##WHERE##.
P.S. Try to pass null instead parameters if you don't want to use them. If null doesnt work try to pass new QueryDataParameters() instead: 
ConnectionHelper.ExecuteQuery("somequeryname", new QueryDataParameters(), where)
Take a look ExecuteQuery Overloads