Hello Rosemary.
If after entering the amount you want and other details, you click the “generate” button and are not seeing those changes reflected in the shortcode, then there’s most probably a JavaScript problem in your installation that’s causing the button generator to not work.
I’m guessing it’s probably the theme. Could you try, as a test, changing the theme to the default TwentyEleven and using the generator then?
In any case, you can edit the shortcode by hand if you wanted. All the shortcode attributes are explained here: [hilite path]Dashboard -› s2Member® -› PayPal® Buttons -› Shortcode Attributes (Explained)[/hilite].
And if you wanted to create a button over at PayPal, that works with s2Member, here are the instructions for that: http://www.s2member.com/kb/using-paypal-created-buttons/
I hope that helps! :)