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 from the Chrome Web Store directly at this link.
To install Capture for JIRA in Chrome, follow these instructions:
Download the extension file.
Open the Chrome Extensions page by opening the wrench icon menu and selecting More Tools > Extensions.
Drag the downloaded extension file from its folder into the extensions page to install.
You should now be able to see the Capture for Jira icon for the browser extension at the top right-hand side of the browser.
For more details about Google's installation policy, see http://support.google.com/chrome_webstore/bin/answer.py?answer=2664769