Hi derDeja,
Sorry, but I can't help with how to follow these instructions in Mac OS 
But if you have the Japanese plug-in installed, there should be a directory called "mecab" within the directory that you installed Anki. Inside the "mecab" directory is a /bin/ directory, which contains the mecab.exe file that you need to run to split the file.
An example in windows would be simply to do something like:
Then inside that directory (bin), you need an input.txt file, which contains Japanese text (saved to UTF-8), then you can run something like this from the DOS-Prompt:
So that the title list of commands looks something like:
So you'll need to research how to a) change directories through your Mac OS commands prompt (I think the "cd" command works?), and b) find out how to run programs with arguments (Again, it might be exactly the same, simply running mecab.exe -O etc etc through the prompt).
Sorry if that doesn't help much but I hope it does!