SPF
Add include:servers.mcsv.net to your existing SPF record. Do not create a second record: two SPF records mean neither works, and that is the most common mistake in this whole area.
DKIM
Found in the Mailchimp account, under Website, then Domains, or in Mandrill's settings for transactional.
Mailchimp authenticates the sending domain with a CNAME pair. Mandrill, the transactional side, has its own separate configuration.
Mailchimp and Mandrill publishes keys under a Mailchimp-provided selector, which is what an external check looks for.
DMARC
DMARC is the same regardless of provider, because it is your instruction rather than theirs. Start at v=DMARC1; p=none; rua=mailto:you@yourdomain.com, read the reports, authorise everything legitimate, and only then tighten. The DMARC guide covers the order, and the order is what stops you blocking your own invoices.
Worth knowing with Mailchimp and Mandrill
Mailchimp accounts frequently outlive the campaigns they were created for. If you no longer send newsletters, remove the include rather than leaving it consuming an SPF lookup.
Watch the lookup count
Every include costs one of your ten SPF lookups, and so does every include inside it. Adding a third or fourth sender is where domains typically cross the limit, after which the whole record fails silently. See the lookup guide before adding another.