Installing guest additions is very easy. Once you start your guest OS, you click on the "Devices -> Install Guest Additions" in the xVM. Then it will mount a CD ROM for you. Once the CD ROM is mounted, just let it auto run or you can run the appropriate shell script from the mounted CDROM. Usually it is mounted under /media/cdrom. So doing an ls under that directory should tell you which shell script you need to run.
To create a share, go to "Devices -> Shared Folders". Click on the add share icon (the one that has "+" in it). Select the folder you would like to share with the guest and then give it a name. It is preferable to give a short name for the share. Let us say you give the name shared-folder. Depending on your need, you can mark it read-only. If you would like to make the share permanent across multiple invocations of the xVM, you can click on the "Make Permanent" too. Once this is done, click on "OK" and close this box.
From your guest, give the following command (I am mounting under /tmp/shared-folder) as root:
mkdir /tmp/shared-folderThats it. You can access all the files under the shared-folder shared from the /tmp/shared-folder directory. BTW, there is no need that the share name and the mounted directory name should be the same.
mount -t vboxsf shared-folder /tmp/shared-folder
For details, please refer to the user manual that comes with the xVM.