Have you read the Wiki? http://wiki.fireflymediaserver.org/NSLU2_Installation
If you look at the network you will see that the slug has a shared directory called either “Disk 1” or “Disk 2”.
Connect to this and create a directory for your music (e.g. music)
Disk 1 on the computer is /share/hdd/data/public to the slug. Similarily, Disk 2 is /share/flash/data/public
Don’t worrty about these differences.
The path you enter into Firefly’s config page is:
/share/hdd/data/public/music if the HDD is connected to USB port 1
/share/flash/data/public/music if the HDD is connected to USB port 2