Hello,
Variations are the way to go to add text fields to a button. To create a text field you would name a new Option Name for the product on the variations screen, but don't provide any values for that option at all, as described here:
http://www.e-junkie.com/ej/help.variants.htm
Once you have done that be sure to update your button code in order to add the new text field that has been created for the button. Most changes to a product do not require you to re-do your buttons, but changes to variants and variations are an exception because we code them straight into the button itself.
