I am using the PHP sample to add contacts from my website. I have uploaded all the files that are there, I put in my username (which I'm not even sure is working), API Key, Consumer Secret. When I hit the authorize button, it takes me to log in to Constant Contact. When I enter those credentials, I get "the client identified by : xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx does not have pre-registered redirect uri!"
Sorry for any frustration that is causing. It sounds like you don't yet have your redirect URI specified. To specify the redirect URI for your account, you would click on the "API Key" link under the "Developer Network" tab while on developer.constantcontact.com. Then you would verify your Constant Contact username and password, and the following page contains your API settings, including the redirect URI.
Once you set that up, you would follow the instructions here to finish setting up authentication.