There are multiple methods you can use to fix this issue:
1) you can use the menu option Tools->Explore Folders->Game System Data Folder. This will open a finder window in the new gamesystem folder location, and you just need to copy over your data files into that.
2) you can open your own Finder window and use Cmd-Shift-G to open ~/Library/Containers/com.LWDTechnology.HeroLab/Data/Library/Application Support/Hero Lab/data. Because this directory is normally hidden, you cannot navigate to this directory without using the shortcut.
3) Included in the most recent Mac installer, or available at
https://www.wolflair.com/download/free/HLDFC.zip is a simple utility - the "Hero Lab Data File Copier" - that will copy over data files from the old location to the new location.
Once the data files are copied over using any of the above methods, no other changes should be necessary, although if you are editing these files within the editor you should ensure that you are editing the files in their new location.