In order to add Payzing to your Shopify Stores Alternative Payments tab, make sure to view the five steps listed in your guides - app.payzing.io/dashboard/guides/shopify (whole process takes about 3 minutes).
If you start form the first step, to install the payzing app to your store, select ‘Install App’ as shown below, however does not yet add Payzing as a payment option (see step 3):
Afterward, for Step 3 select 'Add Payment Gateway' as shown below - this adds Payzing to your payment options under Alternative Payments within your Shopify Store's Payment Settings.
Once these two have been selected, Payzing will show in your Shopify Dashboard under Settings > Payments > Alternative Payments:
From here, you can select to turn on Payzing and input your Uphold username and unique Payzing Gateway Key (found in your Help Guides):
Feel free to let each out to email@example.com if any questions need answers.