You can do this in two ways:
1. Candidates Tab -> Select Candidate -> Action Strip -> Click on Add Panel -> Assign Panel Window -> Select Panel -> Assign
2. Go to Assessments Tab, select an assessment - go to Candidates view, select the candidate and assign panel on the Action Strip
Assessment Tab -> Select Assessment -> Click on "Candidates" on left panel -> Select Candidate -> Action Strip -> Click on Add Panel -> Assign Panel Window -> Select Panel -> Assign
