Emails sent from the Job Add (for instance to several candidates) do not enable the merge fields list but you can still use them.
To do so, you can either, go on the Job Add > People section > Select your candidates > Click on Actions > Select Messages to x candidates > Use a template email that you will modify.
Or you can go to a Candidate page > Select your merge field(s) > Write your email and paste it on your email to several candidates.