1. Go to the link below and login with your Positive Grid account.
https://member.positivegrid.com/presonus-spark
2. Fill in the SN# and click Submit, the activation code should be listed below. You'll also receive an email with the same code.
(SN# can be found near the USB port on the back of your Spark amp.)
3. Go to PreSonus' website and signup for an account.
4. Once logged in, find "Register A Product" on the page and fill in the activation code obtained in Step 2.
5. Once registered, you'll find the download link under your account's registered product page.
See also:
Basic Setup Guide for Spark & PreSonus Studio One