1. Go to the link below and login with your Positive Grid account.
https://account.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 sign up 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