Photopia is a work of interactive fiction or IF. To play IF, you normally need two things: a game file, and an interpreter to play that file. (The common z-file format has many interpreters for various platforms.)

The archive for Photopia linked above lists several files available for download, including several that package an interpreter along with the game file.

Therefore, if you use Windows, the easiest way to play the game is to download the photo201.zip file, extract (unzip) its contents, and then double-click play-the-game.bat.

To play it on a Mac, you'll want to first install a z-code interpreter on your machine. I recommend spatterlight (spatterlight-0.5.0.zip). Download, extract, and run. Next, download the z-code game file and open it with spatterlight. From the download page linked above, you'll want photopia.z5, the second one listed -- version 1.22.