from experience yesterday... the e channel enrolment office is to the left hand side bank of immigration booths, and it located directly before baggage reclaim 5 I think (might be 4, but it's one of those). ifmyoumwere coming from elsewhere you'd find carousel 5 and then turn left.

what I didn't realise (and I got caught by this) is that passports with less than 6 months validity become ineligible for e channel. it allowed my to enter the first barrier, but on taking the fingerprint it rejected and an officer had to come get me.

with less than 6 months you have to fill out a form and enter HKG the normal way. thankfully there were no queues on arrival.

apparently I could register my other passport, then transfer the e channel back to my preferred passport when I get it renewed... although I wasn't entirely sure that the officer knew what I was asking (I'd much rated have it on the passport I frequently use).