To use shared folders, VirtualBox guest additions have to be installed within the guest OS. However, it is cannot be done for Android as mentioned in the following SO thread: Getting VBox Guest Additions for Android x86
Hence, you cannot obtain access to your shared folder within the Android OS installed under virtualbox
One way to transfer files between Android in VBox and external resources is to use internet. You can create a free account on file sharing websites (e.g., deposit files or the like) and then upload files from VirtualBox and download them elsewhere, and vice versa.
