Settings section. Host: smtp.mandrillapp.com
Port: 587
SMTP Username: any username will work - try "Coopdevs" for example
SMTP Password: The key you just have created.
In the Outbound section you can see the latests mails sent and its state. However, it only let you expand information if the mail was succesfully sent. In order to debug sending problems, you must go to API logs (within Settings section) and check the Succeeded API Calls.
Search the machine using its IP and expand log clicking on view full link. The log have the message information and the response from Mandrill.
status: unsignedIn case that the response from Mandrill is that the mail was reject because it was unsigned, you need to setup the correct DNS entries for the subdomain you are using to send the email.
Both DKIM and SPF records are simple DNS TXT records. In order to get the content, please search the subdomain at [Settings->Domains->Sending Domains] and follow instructions shown there.