I can’t figure out where the entry to the Forum is. It’s huge. I tried going a week ago but I hadn’t booked ahead online.
First question: have you since purchased a ticket online? it looks to me like you can't do that onsite. Per the website:
"The purchase of tickets to visit the Parco archeologico del Colosseo is possible exclusively online through this web page and that of the Official Concessionaire CoopCulture: the PArCo has not activated any onsite ticket sales."
"The access to all of the areas in the PArCo will be possible only by booking the entrance online."
if you DO have a ticket, it looks like you might have to enter through the Palatine (Via di San Gregorio 30). It notes on the website that the Forum's Largo della Salara Vecchia 5/6 entrance is currently used only as an exit.
The entrance I have always used is just up the street from the Arc of Constantine. Maybe 200 meters south of the arc. I haven't been there since 2018 but that was a main entrance/ticket purchase point in the past.
The entrance/ticket office to the Forum is (or at least used to be) on the Via Sacra, which you take from the Arc of Constantine. If that has changed recently I don't know, but I doubt it.
The entrance to the Palatine, is (or was) on Via San Gregorio, about 200m south of the Arc.