There is a fairly large parking lot on Historic Decatur Road just to the south (more or less) of the Public Market building. It is between Barracks 19 and the area where Stone Brewery and the Public Market are. That should be a good spot, as it directly adjacent to the Public Market. It even has a few electric car charging places in case anyone has gone green.
If you are approaching Liberty Station via southbound (more or less) Rosecrans, just turn left on Dewey or Roosevelt, then left again on Historic Decatur and look for the lot on your left just north (more or less) of Dewey after you pass Barracks 19.
If you are approaching Liberty Station via westbound Harbor Drive (coming past the Airport), turn right on Laning, right on Cushing, then left on Dewey, right on Historic Decatur and look for the lot on the left. The lot is across the street from the big Ace Hardware lot.
By the way, if coming from the north via I-15 / CA-163, allow some time. It took me 45 minutes to get from I-15 at Mira Mesa down to the Zoo tonight, leaving at 6pm. That is pretty much standard. I don't think I-5 is much better. But, the food at Liberty Public Market should be worth it. Also, note that unless things have changed, some of the stalls may close at 7pm.
Edit: Looks like they may have moved the closing time to 8pm. That is good; when they first started it was earlier.