Are you getting an error that the API key is invalid or some other error?
Can you make sure that there aren’t any extra spaces on either side if you’re pasting the API key? If that still doesn’t work, if you set the WP_DEBUG constant (defined in your wp-config.php file) to true, you’ll see some messages added by the plugin in key parts of the process for the logs.
the SSL certificate http-request response is invalid due to an invalid certificate error (we should catch this as an exception and log it instead or maybe be a bit more verbose to the user why this happens), I added:
to the file wpmandrill/lib/mandrill.class.php after line 429, solved my problem. 🙂 I applied this on my dev env, I will check again once I deploy the original mandrill plugin on production to make sure it works there as expected.
I’m running into this issue too in my local dev environment which is a Windows 7 PC running the XAMPP stack with PHP 5.3.8. In our staging environment which runs Centos and PHP 5.3.18, the same Mandrill API key was accepted without a hitch. Both WP installs are similar (local dev has a few extra plugins) with WP 3.5 and wpMandrill 1.23.