Try this

var searchAppointment = from app in entities.Appointments
                                        join user in entities.Users on app.UserID equals user.UserID
                                        where app.AppointmentDate >= dtStartdate
                                        && app.AppointmentDate <= dtEnddate
                                        && user.UserName == userid
                                        orderby app.PatientFirstName, app.PatientLastName
                                        select app;