This has been tested on Linux host machine, for Windows consult VirtualBox user manual. Link is posted on my lab schedule under resources.
1. Create a folder on your host machine.
2. Before the machine is running, access its settings and navigate towards "Shared Folders".
3. Click the icon to create a new shared folder, locate the path to folder you just created in step 1. Name the folder as you want it to show up on guest system. Make sure "auto-mount" and "Make Permanent" options are activated.
   
4. Click OK, and start up the virtual machine. Once inside the VM navigate to top menu options, Devices -> Insert Guest Additions CD image.... Click on it and you should see CD image titled "SR0" appear on Puppy desktop.
   
5. Click on the "SRO" CD image, and then click on autorun.sh program. Program will open up the terminal and start running, once it's done it'll prompt you to press ENTER.
   
6. You should be all set. Your shared folder should be created under /media/sf_[YOUR_FOLDER_NAME]. Try creating some files both on host and guest to make sure it's working.

Author: Anton Njavro.