In Chrome version 21 (stable release date July 31, 2012), Google implemented a new policy around installing extensions that are not available in the Chrome Web Store.
This policy affects the Capture for JIRA Chrome extension and blocks direct installation of Capture for JIRA from its current server at downloads.atlassian.com.
To download the Capture for JIRA browser extension, navigate to Capture (menu in the JIRA header) > Get browser extension. Alternatively, you can download it directly from the Chrome Web Store.
To install Capture for JIRA in Chrome, follow these instructions:
For more details about Google's installation policy, see http://support.google.com/chrome_webstore/bin/answer.py?answer=2664769