This plugin isn't bundled with any XWiki product by default.
Plugin contributed by Balsamiq Studios - Peldi. This plugin can be downloaded.
BalsamiqMockups Plugin
Using a wiki for software or web site design makes your team more productive. Add the ability to create, edit and embed UI Wireframes created with Balsamiq Mockups and you'll save even more time and money. On top of that, with quick and easy iterations, your software or web site's usability will be much higher, resulting in happier customers and ultimately more sales.
Just click on "Add UI Mockup", edit your mockup, name it and save. The mockup will show up as an image on the page, with an "edit" link under it. Every version of the mockup is saved as a page attachment, both as an image and as clean, human-readable XML data.
Download Balsamiq Mockups for XWiki and give it a try on your own XWiki installation, for 30 days, free of charge.
More info can be found on Balsamiq Mockups for XWiki homepage.