You have to manually click on the little star next to the job in your jobberBase admin panel.
If the job is currently inactive you will see it on the admin home page.
Otherwise you need to select the category from the admin menu. To do so click on one of the submenu items that show up when you hover over the "Categories" menu item in your jobberBase admin panel.

How do you know what category contains the job you want to make featured? You need to check categories one by one until you find it, and than just click on the little star and it will turn yellow.

It would be useful to have the category name showing in job posting e-mails out of the box, found the fix long time ago for older version, but not yet for new one:
Not sure if it will work on 1.9x :http://www.jobberbase.com/forum/topic2133-category-name-displayed-in-admin-emails.html

This would make it easier to track down jobs.