i have my mac set up in a slightly different manner and it works great.
two nics with the Macs on the external network
using the admin log in to my router, I assigned the LMCE machine and the two Macs static IP addresses.
then when I want to transfer files, I open a finder, click on the 'Go' in the menu and then click 'connect to server'
At the prompt enter smb://192.168.1.35 (that IP address is just an example, you must use your LMCE IP address, so smb://whatever_your_ip_is)
you should then be prompted for a password and then for which folder you care to use (public, user1, user2, etc...)
After choosing the folder you will see the LMCE file structure. Add files as appropriate.
I used this method for several reasons:
1. My Macs are not dependant on the LMCE machine to connect to the Internet
2. I had trouble getting the LMCE port forwarding to work properly when using the internal network which made Bitorrent downloads a pain.
3. As I had to specifically tell my Mac which folders could be publically accessible, it still would have required moving around the media in my Mac (or changing permissions)
4. I just like having the media locally on the media PC.