CCPNmr Analysis is probably the best NMR assignment software available and I strongly recommend using it. For details on how to download and install it (the current version is 2.2.2), please see the CCPN downloads page.
I set this page up when no detailed manual for CCPNmr Analysis was available. In the mean time the wiki-style manual is fairly comprehensive. But you might nonetheless find these page to be a useful introduction to the basic functionalities required for protein NMR assignment.