Read the manual. There are different ways but some need to have the dip switches set to award either Game or EB or Score.

http://mirror2.ipdb.org/files/1565/Bally_1981_Medusa_Manual.pdf

Page 6 section C - Olympus Bonus - This depends on what special is set to award.

Page 7 section G - Extra Ball Feature

Page 7 section K - Special - Replay/EB/Novelty settings

You need to complete the drop targets on the right..and the standup targets on the left. When you complete each set..the 'timer' (0-9) will stop on a number (in the middle of the PF). You then need to complete the 'other' set and match the 2 numbers

Basically it a timing thing. It seems the drops are easier to hit. So if I were attempting an extra ball..I would probably start with the standups on the left and complete those and lock in that number. Then try to time the last drop target to match that locked number

On the play field is a display with numbers that cycle thru. When you complete either set of 3 targets the number on that side of play field will stop, now you need to complete other set making last target as the number from that side stops on same number you finished other side on.