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Go to Settings -> General and fill out the "NGP API Key", "Donation Support Phone Line", and "Addt'l Information for Donation Footer" fields.

"NGP API Key" is how this plugin authenticates with your NGP VAN service. You need to make sure the API Credentials string you get from NGP is for the "CWP" API.

"Thanks for Contributing URL" is where the contributor is sent after a successful contribution.

"NGP Secure URL" is used if you need to specify a different secure URL than the site might have already been configured to use (for instance, your SSL cert is for donate.yourdomain.com, which points at your WordPress site, but the site also loads under http://www.yourdomain.com).

"Donation Support Phone Line" is shown with the error message when the contribution gets rejected by the NGP VAN server.

"Addt'l Information for Donation Footer" might be used for listing things like donation mailing address, donation limits, taxable status of donations, etc.

"Check to accept Amex" is a checkbox that, when checked, causes the American Express options for card-type to show up on the front-end.

If you want to work in a dev environment, use ".dev" as the TLD (We develop in virtual hosts on our developer machines. Edit /etc/hosts to add a .dev url). When this plugin is live, it forces SSL. Make sure $_SERVER['HTTPS'] is set, valid and "on".