Syntax clumsy but at least the formula is built-in.

a = Quantity[4*10^14, "Hz"];
b = Quantity[273, "Kelvin"];
FormulaData[{"PlanckRadiationLaw",  "Frequency"}, {QuantityVariable["\[Nu]","Frequency"] -> a,  QuantityVariable["T","Temperature"] -> b}]

I(\[Nu])==2.728045498776555*10^-37 kg/(s)^2

Thanks for the quick response. That way of doing things makes sense, but it was a lot easier the old way. I suppose they had their reasons for changing things, but it seems to me that at least leaving the easier, more straightforward way would be a lot better. For instance, can you think what it would look like to use that kind of syntax to calculate the black body radiation formula. Anyway, thanks. Your response really helped.