Cannot install guest additions

[Asja]

Host: Windows XP
Vbox: 4.0.2 r69518
Guest: Linux 2.6

I am trying to use shared folders and it says I need to install Guest Additions.
I then click Devices->Install Guest Additions and the following error is presented ([path] is my local path to the iso):

Unable to mount the CD/DVD image [path]VBGuestAdditions.iso on the machine Android Mobile v0.3. Would you like to force mounting of this medium?
Could not unmount the currently mounted media/drive (VERR_PDM_MEDIA_LOCKED).

Details:
Result Code: E_FAIL (0x80004005)
Component: Console
Interface: IConsole {515e8e8d-f932-4d8e-9f32-79a52aead882}
Callee: IMachine {662c175e-a69d-40b8-a77a-1d719d0ab062}

The iso file does actually exist at the location specified by path.

I'm trying to be verbose here because I haven't really used Linux before, nor have I used Virtual Machines before. The posting guidelines mention VBoxManage, but I just get "not found" when I type that in.

Anyway, help would be appreciated. Of course if there's another way to get a file onto the virtual machine without using networking/usb/shared folders then that would be good enough as well. Thanks.

[fixedwheel]

Could not unmount the currently mounted media/drive (VERR_PDM_MEDIA_LOCKED).

unix like operating systems lock the CD drive when the medium filesystem is mounted: unmount it from within the guest

[Asja]

After unmounting, when I click "Install Guest Additions" nothing happens.

[fixedwheel]

i guess you should do some more than just only click. What do do depends on distribution, see VBox manual and forums Howto: HOWTO: Install Linux Guest Additions + Xorg config