Discounts you configure in E-junkie would need to be applied in the buyer's E-junkie cart before they proceed to checkout, not during checkout on PayPal's site. The Discount Code field will only appear in the E-junkie cart if items in the cart or the whole cart order itself would qualify for a discount you have set up in your Seller Admin.
Product-specific discounts can only alter the per-unit price of the product, not take a single, flat amount off the order total nor a percentage (though you can figure what that percentage of the full price works out to, and set that as your discount amount). Discounts for All Cart Items can take a single, flat amount or percentage off the Item Total (pre-tax/-shipping subtotal) or Shipping & Handling charges.
For testing purposes, you can create a secret Discount Code that takes 100% off the Item Total, so you can enter that code in your own cart to place a test order yourself without going through PayPal. Since this discount would be applicable to any cart order, creating this discount would also cause the Discount Code field to always be displayed in the cart for all your buyers. This help page gives other tips for testing various aspects of your E-junkie setup: