Antibiotic is actually a prevention/cure of any secondary bacterial infection. It cannot cure the flu itself. Since it's just a kitten, your vet may not think it necessary to burden the kitty with it since there can be side effects such as nausea, loss of appetite, etc.

Best way to help yr kitty is help it clean its mucus and tear stains by wiping with a warm towel. Also putting it in the bathroom with the hot shower running (keep it away from the water though) may help to clear its blocked nose. Make sure your kitty eats/drinks enough. Feed it some canned wet food.