Edit: Should have read your post better.... You should be able to include the javascript "as is" in the post footer at Atahualpa Theme Options -> Post/Page Info Items
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Other Option:
The theme is prepared for the plugin Sociable
http://wordpress.org/extend/plugins/sociable/
If you install Sociable you can place a (small) digg button in the post footer. You set the buttons and their order at the Sociable settings page, and include them in post footer via Atahualpa Theme Options -> Post/Page Info Items -> %sociable%
AFTER you installed Sociable there will be some additional info at Atahualpa Theme Options -> Post/Page Info Items on how to set the Sociable settings to improve its visual integration with Atahualpa
If you wanted to include a HUGE digg button, there must be a plugin specifically for that, I have not used any yet.