Welcome to the forum.
a) is grammatical though unlikely. Why don't you tell the person where you do want to meet them?
b) is not correct. It's a double negative. You used "You didn't tell us" with "neither/nor". You can say either:
- You informed us of your absence neither by phone nor by email.
- You did not inform us of your absence either by phone or by email.
Interested in Language