I'm not keen on heating them in the microwave from cold, because no research has been done on the changes it causes to the ingredients, and it definitely changes the final texture.
But this works beautifully: fill your big mug with boiling water, and leave it to heat the mug for 30 secs or so (like warming the teapot - anybody know what I'm talking about????). Empty the mug, and add the contents of the tetrapak to it, then fill the empty tetrapak carton with boiling water (or hot black coffee) to gather all the lovely left-over bits, and add to the mug.
It really should be hot enough to drink, but I'm guessing that up to 10 secs in the microwave might not hurt....
Mmmm, what would I do without Chocolate Velvet?