If you want to use the VB shared folders feature, here's the short version. Guest additions must be installed and compiled into the kernel.
Create the folder that you want to share on the host machine, say, ~/vboxshare. Make sure the folder permissions allow sharing.
In VirtualBox, select the machine to which you wish to share the folder and go to Settings-->Shared folders. Add that folder. I made mine "permanent." The folder name (say it's "vboxshare") should now show in the settings for that VM without the path: vboxshare. Put a file in it for testing.
Fire up the Virtual Machine. Once you are logged into the virtual machine,
- Create a mountpoint as root: mkdir /mnt/foo.
- Mount the shared folder as root: vboxsf -t vboxshare /mnt/foo.
- Test: dir /mnt/foo. The test file you put in ~/vboxshare in the host should show in response to dir /mnt/foo in the VM.
This is the capsule version of the directions in section 4.7 of my copy of the VirtualBox manual
. Instructions for installing Guest Additions are in section 4.1.1.