I'm having the same issues. I'm having to use Constant Contact plugin version 2.3.1 because our WordPress site is using version 2.9.2. There are no error messages with the installation.
The following message appears everytime I try to get into the settings:
"This plugin is not configured with a valid Constant Contact account.
Please enter a valid username and password then press the Save Changes button before continuing."
I've had to ask our Constant Contact admin to reset our password once already and I'm afraid it will get locked as everytime I try to change the settings for your Constant Contact plugin, it attempts to login with the password. I know the user name and password works because I've used it on the actual constantcontact.com website.
Is this an issue with our verions of WordPress? I'm having a hard time convincing my boss to upgrade to 3.3.1 due to the possibility of the website breaking from currently installed plugins.