For category restrictions do the following:
From your coupon admin
(Gift Certificates / Coupons > Coupon Admin )
Create and insert your new coupon (save it). In this first step, do NOT select anything for the drop down "restrictions" box -- just create the other variables for the coupon and save it.
Next, you'll see any coupons you have created listed in the admin screen.
To the right you'll see a list of buttons.
One of those buttons says "Restrictions".
Select the coupon you want to add category restrictions to, and click the "Restrictions" button.
You'll be brought to a screen where you can select the categories etc.. that the coupon should only apply to.
Select your restrictions and "Add".
You're all set.