For anyone interested, here is a fix for the map downloader not putting the mapDownloads and maps folders in the right place. Can be added as a jarmod in MultiMC after ACBin.jar or the 2 jars can be merged together.

Lastly the mapInfo.txt needs to have valid links (data within must be separated with both a comma and a space eg: ", ")

Tested working with a personal mirror of the old maps I have.

All I did was edit the AC_GuiMapDownload.class in Notepad++ to change the ./maps ./mapDownloads and ./mapDownloads/map.zip to ../maps ../mapDownloads and ../mapDownloads/map.zip respectively.

Lastly I updated the leading byte before each of the 3 strings to match the new lengths. (Using the Hex Editor plugin for Notepad++)

ACBin-MapDownloadFix.jar (Dropbox)
https://www.dropbox.com/s/p53dkdtzodbm06e/ACBin-MapDownloadFix.jar?dl=1

Additionally, here's an updated MultiMC instance which includes the fix and also restores the Quit Game button on the main menu.

AdventureCraft r1095 [MultiMC].zip (Dropbox)
https://www.dropbox.com/s/ryoylp1o56zx1z0/AdventureCraft%20r1095%20%5BMultiMC%5D.zip?dl=1

Note: I have not included my mapInfo.txt file, the one in the download is the original from r1095.