I've made a custom function which works fine, but I've started it:

Function factorial (x as Integer)

so presumably it ought to be invoked every time I type in =FACTORIAL(B3) in to a cell, for instance.

As it happens, this returns #NAME?, and the function only works if I type it in as =PERSONAL.XLS!FACTORIAL(B3)

why?

It's currently saved in a module in Personal.xls - should I put it as an add-in, and if so, how?

Thanks in advance,

Anthony