Ok, thank you for trying, but I've already done all the steps described in here:
Forum VirtualBox (Step guide for installing guest-additions) *cannot post the link yet, so new user, sorry*
The thing is that I use KDE for my research and development of websites. Sorry, but in the forums there's also not a question like this one and I'd like to try to see if there's something any of the geniouses around Oracle could help.
The main thing is that you can install Kubuntu as a guest, you can log-in for the first time without any problem(excerpt for the sound and shared folders and USB), but after you do the update required to install the prefered (Lampp, Konqueror, Guake, flashplayer plugin, and etc), Kubuntu does loses everything you've done. I mean, there's no X11-guest(-hwe) that can work with DKMS or the VBox-guest-dkms-*-*(-hwe) or anything. There's no repository or support from Kubuntu and after tryed re-installing Kubuntu for over 4 times trying new conf and adviced ways from Internet to implement the guest-additions I couldn't make them work(No mouse, no keyboard, no X "usefull" indistinct if is AMD or Xserver or X11, no sound, no shared folders or USB). In sume, you see how Kubuntu logs you in but after that, you can only tell to VBox to shut it down from the VBox menu.
It would be lovely to get into my Kubuntu again, even if it doesn't emit a sound, but hopefully within VBox you can get it to work with all the others useful and lovely inputs. Thank you very much in advance.