Are you just trying to share files with the Windows guest? Have you configured Virtualbox for sharing the directories, in "Devices => Shared Folders?" That is necessary for the guest to see the shared directories, although there may be other ways to set up the network. Then, if it doesn't work, you may need to install Samba on Ubuntu - I'm not sure if it's installed by default or not.
For more information on configuring the samba server on the host, you can take a look at this article. Usually, all you need to do is edit
/etc/samba/smb.conf to add the name of the computer and the network, which defaults to "Workgroup", but should match the name of the windows network. These two lines in
[global] are the ones that set it:
workgroup = workgroup-name
netbios name = server-name