If the Buy Now tab is greyed-out on the button codes screen, that means your product is configured with settings that won't work with Buy Now buttons: Variants/Variations, and "Let buyer edit price".



If you can select that tab, but no code is provided there, first you need to integrate at least one payment processor with your E-junkie account:

http://www.e-junkie.com/ej/help.payment.processors.htm