Donation checkout flow not being displayed in config

Please confirm that you have successfully completed step 1 in that section:

Before completing this configuration, create or confirm that you have created a checkout flow that uses this modules's donation_multistep_flow plugin. Place the "Add Donation" pane in the first step of the flow, just before the Donation step.

When you browse to /admin/commerce/config/checkout-flows you should see the multi-step checkout flow that uses our plugin.

If you want to use the mixed flow, and add donations outside the checkout flow, some custom work will be needed as I only have that feature for the donations only flow at the moment. The core task is to create and configure for the customer an instance of our Donation order_item. You might look at improving src/Plugin/Block/QuickDonationBlock.php and \Drupal\commerce_donation_flow\Controller\DonationController::quickDonation so that it chooses the proper paths for redirection or build a route of your own that does this task for your setup.