The zip does not contain a module manifest, so you can't install as a normal module.
Just follow these easy steps:
1) Download and unzip locally
2) Copy DNNXsltDbRepository.dll to the Bin folder of your DotNetNuke
host.
3) If you don't have XSLideShow installed, copy XSLideShow.cs to your
App_Code\XsltDb folder.
4) Copy both jdownload and ui folders, with their content, to your DNN
host under /Portals/_default/js/. If you wish to move js and css files
elsewhere, just adapt the paths in the xslt.
5) Create a new XsltDb module instance in a page of your choice, open
the .xsl file with a text editor (e.g. notepad) and copy its content
into the XsltDb editor. Save and publish.
6) Open your new module settings, and there select the Repository module
and category of items you wish to show.