You can fly direct to Parintins from Manaus, and from anywhere else via Manaus. The flight should cist around R$600. You can get a boat from Manaus (15 hours) or from Santarém (6 hours). During the festival, just to get you in the mood, special party boats are organized to go down to the festival and (if you have any left) back...
Jau National Park, Amazonas. Massive and splendid national park 220 knm up the Rio Negro from Manaus. A number of Manaus-based agencies organize tours lasting 2-5 days to Manaus. Because of the logistics it is best to book in advance and build other bits of your itinerary around such visits (same is true for any national park). Agencies offering tours include: Eco-discovery Tours, Brazil Ecotravel, Southern Cross Expeditions and Anavilhanas Ecotours. Compare prices and the quality of the guides and boat accommodation before booking...
Boulevard Castilhos França, 707 - Campina
Rua Barão de São Domingos, 268 - Centro