I need help computing a pA2 using global fitting.

General information:

This article contains a fairly detailed example, with screen shots.

Troubleshooting fits that don't converge:

If you are using Prism 5.00, update to Prism 5.01 (Windows, Mac 5.0a is fine). This update introduced a new rule for initial values, which is used to obtain an initial value for the pA2. This is a much much better rule than used in previous versions, and will enable fits to converge that otherwise wouldn't. 

What units should I use to enter B? What units is the pA2 in?

You can enter B in any concentration units you want, but molar is standard. The units of 10^(-1*pA2) will be the same as the units of B. So if B is in molar, then pA2 is the negative log of the Kb.