I bought three EZX - It took a little over 24 hours to get an email with the serial numbers. Looks like its a manual process so it won't be instant.
I assume you are talking about the Toontrack MIDI packs. What usually happens is you go to your account on the Toontrack web site (or make one if you do not have one already), Click on "I Want to register my product", enter the serial number when promoted. The product then appears in your account and you can download it.
I am guessing the process is the same with the generic serial numbers, except there will be an extra step where you choose the pack.