Groupe Transair flies twice daily between Dakar and Ziguinchor. It flies less frequently between Dakar and Cap Skirring (currently once per day on Fridays and Sundays).
Going overland to the Casamance from other parts of Senegal is not a simple feat. It requires going around or through The Gambia (meaning you'll have two border crossings), which adds many hours to the journey – particularly since unreliable ferry transport can mean hours of waiting to cross the river (there's no bridge). The most direct route (but not always the fastest) is Dakar to Kaolack, then continuing southward on the N4 (the so-called Trans-Gambia Hwy), crossing into the Gambia at Farafenni, exiting the Gambia at Missira, and continuing from there to Ziguinchor.
Three boats travel between Dakar and Ziguinchor: the Diambogne, the Anguène and the larger Aline Sitoé Diatta. There are currently four departures from Dakar per week (see the latest times and prices at www.cap-skirring.voyage). All are operated by COSAMA. It's wise to plan ahead as tickets can sell out in advance. Unfortunately, you must buy tickets in person, but some guesthouses will do this for you.
One of the few travel agencies that can book ferry tickets for you is Diambone Voyages in Ziguinchor (but they must be booked at least three weeks in advance).