This issue has been logged.
In the meantime, as the sourcePath entries are not going to be correct (or useful) when opening a gallery on a different computer, perhaps the best course of action would be to simply remove them all from the gallery's XML file before opening the gallery in JuiceboxBuilder-Pro on your Mac.
This can be done in a single search and replace action in a text editor which supports regular expressions, for example Notepad++.
Open your gallery's XML file in Notepad++.
Go to 'Search -> Replace...' from the drop-down menu at the top.
Select 'Regular expression' for the Search Mode.
Search for: sourcePath=".*?"
Replace with nothing (empty string).
Click 'Replace All'.
All sourcePath entries will be removed with this single action.
You can also load multiple XML files into Notepad++ at the same time and remove all sourcePath entries from all files at once.
This might save you some time if you have a large gallery (or multiple galleries) that you need to open on your Mac.
Before making any changes to any files, I recommend that you make backup copies first, just in case anything goes wrong and you need to reinstate the original files at a later date.