When I click the option box to compare two equations, why don't I see radiobuttons to select equation 1 and 2?

The nonlinear regression dialog has an option "fit to two equations and compare". When you check that option, Prism is supposed to show you radiobuttons to switch between equation 1 and 2.

On rare occasions when using Prism 3, those choices don't appear, for reasons we don't fully understand. But there is an easy work around. Uncheck the option, then recheck it. Now the equation choices will appear.