star-cd and design , same one or different one ?

Different, STAR-CD is cd's older solver and has lots of multiphysics, moving mesh etc capabilities STAR-DESIGN is a "cut down" package with CAD modelling and more limited physics, it uses the new solver STAR-CCM+ which is a bit like STAR-CD in terms of capability but a lot easier to use.

so CCM+ is more advance, but for simple case , can i just using star design ? eg , modelling flows in a simple room

Yes, if the physics are relatively simple then STAR-Design is fine, if you want to add in other things such as radiation, multi species transport and or more advanced meshing I would suggest moving to STAR-CCM+, it is very easy to use so it isn't much of a step up.

thx my project is to model the flows in a single room with a heat source (1.2kw) placed in the middle of the room, and 2 openings ;1 low 1 up to form the displacement ventilation system. so far i am using star design, problem is how do i apply the heat source ? thanks