Creating shared folder that can be accessed across networks

I just purchased an Airport Express and I am trying to set up one specific folder that can I can share to a friend logging in from a different network. I had it set up and working flawlessly using with the old Linksys router that I just replaced, but cannot, for the life of me, figure out how to configure this setup now that I am using the Airport Express.

I am using an iMac running Mountain Lion and my friend is using a Macbook running Lion.

Any ideas would be greatly appreciated! THanks!

 

I'm sorry, I didn't explain well enough. The folder that I am trying to share is on the desktop of my iMac. I am not trying to directly connect any computer or external hard drive to the Airport.