I wanted to share some weirdness with the Premium MailChimp plugin (the one offered here) and the free widget plugin (offered by MailChimp). It's not a huge issue but might confuse people the way that it confused me so wanted to pass it on.
I had the free widget plugin installed first but then deactivated it and added the Premium plugin after. It immediately said it needed to update but when I did, it triggered a fatal error. I couldn't figure it out and came back here to check. That's when I realized (doh!) that of course I wouldn't upgrade through the typical wordpress interface since this is a premium plugin. What was happening was that my WordPress/BuddyPress install was seeing the FREE plugin needed upgraded and since it has the same name as the PREMIUM upgrade, it was installing the free one, which effectively REINSTALLED it and erased my Premium one. So I deleted the unwanted free one, reinstalled the Premium one and ignored the "update" yellow bar and it's all good. But it took me a minute so I thought I'd pass it on in case I might save anyone else some confusion.