This is how I typically do it, and it is fool proof:
First write out 2 IIf statements:
IIf(conditions, A, B)
IIf(conditions, C, D)
Then copy the second one, and paste it over the top of A.
IIf(conditions, IIf(conditions, C, D), B)
Then fill in the parameters, e.g.
IIf(x=1 and y=2,IIf(z=3, 99...