How can I connect the wire that goes to the grounding rod to the grounding electrode system?
There is no separate ground bar in that panel because it is the main panel. Both grounds and neutrals share the same bar. Just know that if you add a ground bar only grounds can terminate on that bar.

For reference: YES, that is a main panel. It is a split-buss panel. The top twelve spaces are "main" breaker spaces, and technically should only have two-pole breakers. Breaker 9/11 is a "lighting main" and controls the bottom 8 breakers.