Can't connect CDROM from gui

That is odd, I am not able to reproduce this.

Does it also happen with a newly created VM?

Could you please run virt-manager with --debug and see if there are any errors that get displayed when you click the button?

Nothing happens when I click the button. I'll attach the complete output.
At the end, the following errors are shown when switching between 'Console' and 'Details':
  (virt-manager:3055): GLib-GObject-CRITICAL **: g_object_set_qdata: assertion `G_IS_OBJECT (object)' failed
At line 522 I opened the 'Details' window. Then I pressed 'Connect' a few times. Nothing happened and nothing was shown in the output.
At line 524 I switched to 'Console'.

To answer your other question 'does it also happen with a newly created Vm'. Yes, but the wizards asks for a CD image and that works, so I'm perfectly able to install the machine from CD. After the first disconnect, it's not possible to use the GUI to connect cd's to virtual machines no matter whether they are running or not.

There are two workarounds:
- From virt-manager: Shut down the machine, remove the CDROM, and add it again. While adding, you can select an image.
- From the command line using virt-attach

Confirming because I can recreate (on Precise, up to date today virt-manager 0.9.0-3ubuntu1).
Also no output with --debug when clicking the button.

Alvin: I notice you're running Kubuntu, so am I - could be a red herring, but seems worth mentioning.

I have the same problemon 12.04 amd64 and virt-manager 0.9.1-1ubuntu5.1 (precise updates)
Step to reproduce:
1) create a new VM (using an ISO image mounted on the CD device offered in the wizard)
2) when the machine is created view "details" in virt-manager: there is no CD device
3) try "add hardware" in the list there is no CD device to select (btw there is no IDE bus to select - in 11.04 the CD was on the IDE bus)

With 11.04 I had no problem. Then I upgraded to 11.10 and then to 12.04. Now the problem exist
I tried also a fresh install of 12.04: same problem

I need to boot an existing VM with partedmagic to enlarge a unmountable file system, so the workaround to try to attach a CD with the command:
sudo virsh attach-disk VZ02-zentyal /Mio/ISO/partedmagik/pmagic_2012_07_13_x86_64.iso vdc --mode readonly
doesn't work: the disk is correctly attached as virtual disk 3 but when I reboot the VM it disapperars from the boot menu

Noted in the comment of Dace Gilbert.
I'm using two machines to test:
1) HP laptop Kubuntu 11.04 (updated to 11.10 - updated to 12.04)
2) assembled desktop Ubuntu server amd64 12.04 (fresh install + kubuntu-desktop)

Found a very strange workaround.

In this example the VM name is VZ02-zentyal

To reproduce:
1) save a copy of /etc/libvirt/qemu/VZ02-zentyal.xml
2) edit this file with sudo virsh edit VZ02-zentyal
3) add in the device stanza this definition (taken from a VM created with Kubuntu 11.04)

    <disk type='block' device='cdrom'>
      <driver name='qemu' type='raw'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='1' unit='0'/>
    </disk>
    <controller type='ide' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>

4) reboot the VM -- ATTENTION now virt-manager crashes
5) reboot the host system
6) restore /etc/libvirt/qemu/VZ02-zentyal.xml from the saved copy
7) restart virt-manager -- Now virt-manager starts, the VM have an IDE bus and an IDE CD (strangely the /etc/libvirt/qemu/VZ02-zentyal.xml file is untouched and there is no cdrom stanza)