Is there some reason you've chosen to do the install manually instead of using the RockboxUtility to do an automated install ? Then you wouldn't be having any of these issues.
For reference, I believe on a Mac, that iPods get mounted under /Volumes, so in terminal you'll need to type:
cd /Volumes
ls
Pay careful attention to what the "ls" command tells you. One of the folders there will be your iPod. Use that name to change into that directory too :
cd "Whatever_ls_tells_you_your_ipod_is_called"
Then you can use the mv command from earlier in the thread to rename the directory.