Select Devices -> Shared Folders from the virtual machine's menu bar, then click Add New Folder icon, from the Folder Path select the folder at Windows, enter the folder name to use, then click OK. The shared folder should be list at Transient Folders.
Mount the folder from the command line as root:
mount -t vboxsf folder_name mount_point
For example, type in "mount -t vboxsf Music /mnt" to mount Music folder to /mnt.
All files in the Music folder, at Windows, should be available at /mnt now.