Math Macro with arguments

[hugh a. duguid]

I have reviewed the sections on Math Macros, 5.1, etc and still need 
help setting up a math macro with arguments.
Specifically, I'd like to have a summation with the upper and lower 
limits, as well as the argument of the summation empty so that they can 
be filled in as needed.
It appears that I do not understand the math-macro-arg <number> business.
I'm using lyx-1.2.1 on Mandrake 9.0