Hi, Luis! I'm not Cathe but I'll try to answer your questions.
Many, but not all, barbell exercises can be done using dumbbells. If you need to keep your hands in a fixed position then a barbell is often better because the bar will keep the hands stationary. And many heavy-weight leg exercises are better done with a barbell because you can put the barbell along your shoulders, letting the strong muscles of the back take the bulk of the weight.
However, dumbbells can indeed be used in place of barbells for many upper and lower body exercises even if the Cathe workout you are doing demonstrates a barbell. If you are doing regular squats or lunges, simply hold the dumbbells alongside your hips; if you are doing plie squats hold the dumbbells in front of you, arms hanging down and palms facing back.
For upper body drills, again simply use the dumbbells and be careful that your hand and arm positions don't flop around; you need to be a little more mindful with dumbbells and hand position.
As far as "6-pack abs" are concerned, that is a combination of muscle conditioning of the entire abdominal region AND of maintaining ultra-low stores of body fat. Sometimes a person can have very strong and massed-up abdominal muscles but are indeed genetically disposed to carrying a little extra fat in that region that keeps the tendonous inscriptions across the trunk obscured. So it can be true. I've never really thought "6-pack abs" were very important personally; they are more for aesthetics than health.
Hope this helps!
A-Jock
