Did you ever get this to work?
It's easy; allow me to clarify.
The PC directory 'Run/Data' refers to the Mac directory 'Blitzkrieg/Blitzkrieg Data Files/data'. Put your .pak files (scenarios, chapters, campaigns) in there and they will run, at least most of them anyway.
If you want to install a mod, create a folder called 'mods' in your 'Blitzkrieg/Blitzkrieg Data Files' directory (beside your data folder). Within that mods folder, create a folder with the name of the mod you want to install (e.g. 'RTV13b' for the Road To Victory mod), and then within that folder, create another folder called 'data'. Your final file structure should look like something along the lines of 'Blitzkrieg/Blitzkrieg Data Files/mods/ModName/data', and it is in this folder that you should put the .pak files for your mod, and then when entering the game, you will be able to load up the mod via the Mods menu. Alternatively you can put the mod files straight into your Blitzkrieg/Blitzkrieg Data Files/data' folder, in which case they will load up at startup. Be careful of conflicts however.
One small problem however... While most of the scenarios and chapters run well, some do crash when activated and some crash Blitzkrieg before it even loads up if they are in your 'Blitzkrieg/Blitzkrieg Data Files/data' directory. What's worse, none of the mods that include new graphics seem to work. You can load the mod, even start the map, but as soon as you come across a new graphic, your computer will enter one of the worse crashes you will ever see; quitting the finder, the only solution for which is a manual restart. These type of crashes seem to happen when you load up the mod through the mods menu. If you load up the mod directly by putting the files into the 'Blitzkrieg/Blitzkrieg Data Files/data' directory, the crash seems to be less severe; only causing a crash to desktop, although I can't confirm this completely, more testing needs to be done.
EDIT: UPDATE: I played around some more and it seems that indeed while loading a mod with new graphics from the mod menu will produce a severe crash when a new graphic is encountered while playing, putting the mod in your 'Blitzkrieg/Blitzkrieg Data Files/data' directory will only cause a crash to desktop when a new graphic is encountered. When I say graphic, I mean most definitely new models and skins, and probably new textures (I can't say for sure). For sounds, I don't know. If the mod only includes gameplay changes, it will run without any problems as far as I can ascertain.