The source file (story.ni) is an Inform 7 source file. Create a new project with Inform 7 and drop the file in.
For the release file (Derelict.gblorb), to play a work like this one, you need an interpreter program: many are available, among them Gargoyle for Mac OS X or Windows; Windows Frotz or Windows Glulxe for Windows. ~ Inform 7
The release website has a link to the release file, and allows you to the play the game right in the browser. It's the default website generated by Inform 7.