Arguments for Macros

[Misha Gavrilovich]

Hi

I am not able to understand how to define macros with arguments in LyX;
like if I want to define \frac myself;
I do not know how to enter an argument mark; if I follow the Users' Guide 
recommendations and try to enter either 

math-macro-arg 1 

or 

\math-macro-arg 1

it just types text in the macro definition box as text.

I am sorry for disturbing you again, 

thank you,

Misha

PS : I use Lyx 1.1.6fix3