Manage > Advanced Settings > Site settings > Payment ( /admin/config/services/payment )
Choose "Payment method" and click "Add payment method". Then simply choose the provider you would like to enable.
Campaignion supports Paymill, Stripe, Paypal and Manual Direct Debit. If you don't find any provider to enable, ask More Onion support to install the according feature.
Add a specific title (the one you'll see in the admin interface) and a generic one (which your supporters and donors will see)
Provide your API keys (you will get them from your Paypal, Stripe, .. account) and for Stripe and Paymill be sure to match your form keys in the "Personal data mapping" table.
If this process is completed all available payment options will show up in the form builder when creating new donation pages. To see it click "New", then "Donation page", make your way to the "Form" step and take a close look at the field at the very bottom.
When editing this field you can pick the tab "Options" and enable/disable payment methods you've previously created.