You'd be taking the Keikyu train from HND direct to Yokohama station. Well, depending on the train, you'd get off first at Kamata and then change to another train headed towards Yokohama. But I'd recommend you just hop on the one that does not require change of station. Those ones leave HND-Int'l at 8.22p, 8.33p, 8.39p, 8.51p, etc.

Even though you won't be taking JR on 12/29 and hence you won't want to activate the pass until 12/30, you could still theoretically exchange the voucher for the pass upon arrival on 12/29. However, the JR office at HND is closed by the time you arrive. Ditto for the one at Yokohama station. So just exchange the voucher and activate the pass on 12/30 at Yokohama station.

http://www.japanrailpass.net/en/exchange.html