Hi, Alain. I'm new to s2member, but have you watched the videos? I found them to be VERY useful, though I don't recall this being address. Then again, it wasn't my issue.
Depending on whether or not you're running BuddyPress and Multisite, I think you need to edit wp-login.php, wp-signup.php, or register.php. This is from General Options:
Custom Registration Fields ( optional, for further customization )
Some fields are already built-in by default. The defaults are: *Username*, *Email*, *First Name*, *Last Name*.
Custom Fields will appear in your Standard Registration Form:
(
http://studentsoftraders.com/wp-login.p ... n=register )
A Multisite Blog Farm uses this Form instead. It supports Custom Fields too.
(
http://studentsoftraders.com/wp-signup.php )
* With BuddyPress installed, Custom Registration Fields are NOT applicable. BuddyPress themes usually come with their own Registration Form ( i.e. BuddyPress redirects you away from the default Registration Form, over to a special /register page ); BuddyPress also has its own Profile Field Configuration Tool, under BuddyPress -> Profile Field Setup. When BuddyPress is installed, the use of s2Member's Custom Fields is not advised; that is... UNLESS you're using the s2Member Pro Module. With the s2Member Pro Module, Custom Fields will be included in all PayPal® Pro Forms, including even Free Registration Forms generated by the s2Member Pro Module.
I'm not sure if that's helpful, but you did say "Anyone."
Good luck,
Lee