Hmm, that's odd. I just uninstalled Cloudmarks and reinstalled it from scratch. I even deleted my Pinboard password from the keychain. It worked for me without a hitch; I was able to log in to Pinboard and get my bookmarks right away.
I really don't know why it's not working for you, unless perhaps you have a content blocker or http proxy that is preventing connections to
api.pinboard.in (the address of the Pinboard API server that Cloudmarks connects to).
Here is something to try. Copy the URL below and paste it into Safari's address bar, or just click the link if Safari is your default browser.
See what you get. If you are able to access the API server, you should see an XML document in Safari, full of your Pinboard bookmarks.
If you get some kind of error message instead, let me know what it says.
Chul