The URL you have listed doesn't make sense to me. If that is your redirect URI then you should not have the other parameters in it such as access_token and token_type (unless those are part of your own website, but that does not seem to be the case).
Or that is the URL that is returned to you in Step 4 which also does not make sense as the URL that will make the return will also be from idfed.constantcontact.com.
I'm happy to troubleshoot this; however some of the information might need to be kept secret for security reasons. You can reach out to use at webservices(at)constantcontact(dot)com.
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