Click on the check box besides the candidate's name.
You will get an option of "Assign"
You can select the job to be assigned by selecting one of the uploaded job and click on Assign.
You can also assign the candidate to a job from within the candidate details page.
If you click on the candidate's profile, click on the "Assign to Job" button and choose the job you want to assign the candidate to.
A new job can also be added during the job assigning process.