Title says it all - thanks!
I haven't tried it yet, but you can try by copying all files in the savegame folder to its counterpart on the other system.
Win Vista/7/8: %USERPROFILE%\Documents\Telltale Games\The Wolf Among Us
XP: %USERPROFILE%\My Documents\Telltale Games\The Wolf Among Us
Mac: ~/Library/Application Support/Telltale Games/The Wolf Among Us