Stop the virtual machine. Then in the VirtualBox "OSE" main config window you should see:
On the left side, the list of VM's which you have. On the right side a panel with three tabs: Details, Snapshots, Description.
In the left pane select the (stopped) machine.
In the Details tab of the right pane you should see several sections, each with a click-able blue link. The sections are: General, Hard Disks, CD/DVD-ROM, Floppy, Audio, Network and Serial Ports. Click the blue link for CD/DVD-ROM and you should see the settings I mentioned in my previous post.
In you still can't get it working, I suggest you should read the VirtualBox documentation. There is an image of the relevant settings dialog on pae 29 of the user manual, which I found can be downloaded here: http://www.virtualbox.org/download/UserManual.pdf