When you will open that page type instance (page in CMS) in Forms view there will be a porperty of type XForm. When you will click "..." button a XForm seelctor window will open and there you will be able to create new empty XForm. Unfortunately there is no general XForm browser / editor. It's available via page property editor.
Sorry for beeing unclear in my blogpost, but you have to do exactly what vladis said. when you have created your property on a page you logon to editmode and edit the property on that page. When you do that you will get the xformseditor, here you create an empty form and select that empty for for usage. when that is done it is up to you doing the magic with code :)
I found a nice tutorial (because I need to implement similar task - add feedback and contact us information on my site)
But I never worked with XForms. I create simple page type and add XFrom property. But what to do next not quite understand.
Maybe If anyone has a link to the source code would be very grateful.
Thanks in advance.