Sorry for delaying. I saw your comment but I was at the University so I couldn't respond it.
But now, I will elaborate this issue as far as I can.
First the first: normally, If you want a a new plasmoid you need to click Add widgets -> Get new widget -> Download new plasmoid widget. But somehow, in this case this way doesn't work, so use your package manager instead. (simply go ahead to use your Rigo and type 'adjustable clock' in the search bar and install it)
And yes, I've also experienced that disappearing desktop thing since my system has been upgraded to KDE 4.11. (amongst other annoying things)
So, before showing you some witchcraft for KDE I better to tell you what to do when your KDE desktop utterly collapsed
The first aid is quite simple: find your ~/.kde4/share/cpnfig/plasma-desktop-appletsrc file (it's a so long config file, don't be surprised) and delete the entry of the last element (widget) you've added the desktop (for searching use Ctrl-F)
Don't worry, you needn't re-install anything
So, now you have already get your prospective plasmoid app, you need to play a bit with skins and settings (there are a lot, you can simply adjust it just like any other widgets)
But I'm sure it's not enough for you because you are just like me.
So, let's make our own desktop clock by clicking the at the under-right corner of the Appearance panel. (I've chosen the QML format option but the format itself couldn't be vital important I think)
So you can simply edit this HTML/QML contest , save and (if you are lucky
) it will be set on automatically.
Okay, so far so clear, I guess.
If you need HTML examples I suggest you to view codes of the in-built themes. I mean, just download the source of this widget (from here, perhaps: http://kde-apps.org/content/download.php?content=92825&id=1&tan=71059266
), unpack and go to the ./applet/themes/name_of_thechosen_theme/config folder.
I think here is that you are looking for.