The harp plays on only three movements: 5, 7 & 9. Nine is very easy, you just have to count carefully until you come in. Some say 7 is the most difficult because of the constant arpeggios and page turns, but I found it helps to copy the pages and tape them together like an accordion so you can grab the corner quickly. The physical momentum of the figures helps keep you going and it’s beautiful and fun to play.
For me (maybe because I have small hands–I don’t know), the most difficult is no. 5 because the chords jump around and some choir directors like to take it too fast because their singers don’t have enough breath support. The tempo is marked in the music and is reasonable. If they try to rush you, stand up for yourself. If necessary, take a metronome with you to rehearsal so you can make your point. I wish I had last time I played it.
There’s an excellent CD you should get if you don’t have a recording of this piece: get the Dresdner Philharmonie with Martin Flamig conducting; it’s on Capriccio Digital, a 1987 recording. You can really hear the harp.

http://www.amazon.de/Weihnachtsoratorium-Martin-Flaemig/dp/B000025ZLP

Be aware, too, that although it’s scored for orchestra, many places will use just organ, harp and percussion to save money, or they may be very tight for space. Sometimes they’ll add a string quartet. One time when I did it and they had only organ, harp and percussion, I was afraid I’d have to carry the whole thing by myself because the power went out and the organ wouldn’t work. Fortunately it came back on just before the service!

Taping and dropping the pages in 7 is exactly right. Be sure the organ and harp are tuned together like one post said above too. That is very important and can be a nightmare. Also remember to take the music for 7 up to the director and accidently remind the director that in 7 a lot of the arpeggiatos do not start on the beat. If you don’t, sometimes they are very busy and don’t look at your part and start wondering why you are coming in a split-second “late.”