For unpacked extensions, you can run with the command line flag:
<path to chrome> --load-extension=<path to extension directory>
For installing a packaged extension in a .crx file, you can use the “external extensions” mechanism to automatically install from either an autoupdate url or a local path:
https://developer.chrome.com/extensions/external_extensions
Two things of note:
-Because this mechanism is intended mostly for distribution to end-users and not for testing, there is a confirmation dialog that allows the user to opt in to the install.
-We may eventually be deprecating the local path based installs in places where we’re enforcing the “must be from the webstore” restriction, and only support the autoupdate url (which must be the webstore update url) approach.