To configure Fixed-Amount-Off promotions when running Special Promotions extension, go to admin panel:
Promotions→Shopping Cart Price Rules.
Specify data in the “Rule Information” tab as required. You can choose whether to supply a discount coupon code or not.
In the “Apply” field select “Fixed amount discount” and set up the discount amount in the next field as shown below:
Don't forget to test your promotion before starting the campaign.