Where should I connect the green ground wire in main panel next to meter?
Fit a ground bar to the main panel, land it there

While, since you're working in a main panel here, you could commingle neutrals and grounds by landing your new ground wire on one of the neutral bars, it's cleaner to avoid that altogether and install a separate ground bar into the main panel instead for the ground wires.  Since you have a Square-D main panel, the correct part for your situation is a PK12GTA or PK15GTA -- you should be able to find it at a local electrical supply house or order it online for about $10.  Simply mount it to the main panel's cabinet in the holes provided, with the meter pulled for the duration (there should be a set top left or top right -- if not, the install instructions give guidance on drilling a set of mounting holes), then attach the ground wire to the newly fitted ground bar.  Once that's done, have the electric company turn the power back on, and enjoy your new subpanel!

(The previous installer committed a bit of a no-no by using a black wire for a neutral and phase-taping it instead of using a white wire, but that can be rectified down the road if it ever becomes an issue.  I would suggest phase taping it white at this point, at a minimum, to avoid needless confusion due to the previous installer's non-standard phase-taping scheme.)