Can you navigate to http://www.splunkbase.com from your laptop? The message that you are seeing normally means that there is a firewall blocking Splunk's access to the internet.
You might be able to find more information about this error in one of Splunk's internal logs. On the Mac, you will probably find the internal logs under /Applications/splunk/var/log/splunk. The file splunkd.log is good for most errors, but for this I might scan most of the logs. BTW, all of the logs in this directory are indexed in Splunk: you can run searches again the _internal index to hunt for errors.
If all else fails, you can go directly to Splunkbase via a browser and download the files to your Mac. Then in Splunk, choose the option to install the app from a file and point to the file(s) you downloaded...
