subnet_mac.swf is a file included in your download of submachine universe. Read the other file "Mac OS Troubleshooting.txt" and will tell you how to open it. I cannot guarantee it will work, as personally, I have had issue with this version working. However it is an alternative you should try.
Also, i'll say it again, I, me personally, have never had any issues when opening the game using the method I previously described. In fact it is how I play all of the submachine games I have downloaded from Mat. I have never had issues with them either. So I don't know what to say. I wouldn't even know what to look for to figure out what is different between our computers that is preventing you from doing things this way. My only suggestions would be perhaps check your version of flash player and maybe restart your computer. I know for a fact, that left on long enough, Mac will has weird issues that will disappear when turned off and on again.