You have to create a Product Type and pull the custom fields into that.  Then assign your product to the Product Type and you will get the extra field in the product description edit screen.

Be aware that this isn't the same as a Product Attribute, where a customer can pick a colour or size from a dropdown.  The custom fields literally are custom description fields that are then static for a particular entry after being configured.  Product Attributes will be added with v2.0 sometime after March.

I would like to include a custom field in the email templates but not on the store front.  The custom field includes instructions for the user after purchase.  Is there a tag that I can use in the email template to make this field show up?  I think I can figure out how to turn off the field on the store front with CSS.