First, open up VirtualBox, and select your Windows virtual machine. Click on the Settings icon to open up the settings window, and click on Shared Folders tab.
On the right hand side, click on a small button that reads Adds new shared folder. On the drop down menu for Folder Path, choose Other...
Finally, simply choose the folder in your host system (i.e., Mac OS X) which will be shared with the guest system (i.e., Windows). Check Auto-mount option.
Now, the setting is complete. Fire up your virtual machine, and you should be able to see your shared drive as a network drive when you open up My Computer window. The shared drive address should be \\vboxsrv