To message applicants to your job, go to the My Jobs page.
You can use the filters to select which candidates you'd like to reach, or remove all filters and select everyone. With the batch selection tool, you can send a single message to multiple candidates.
By selecting at least one candidate with the selection tool, three actions will appear to the upper right.
Click on "Message" to generate the message dialog box, and click "Send" when you're ready. This will message the candidate(s) and generate an email notification as well.