To install this map properly so you can play it in Half-Life 2 Deathmatch, you're going to want to extract the zip into the following directory:

"C:\Program Files\Valve\Steam\SteamApps\USERNAME\half-life 2 deathmatch\hl2mp\"

Of course, if you installed Half-Life 2 in a different directory, you'll be extracting into a slightly different directory. Additionally, instead of "USERNAME" you should substitute what user name you're using for Steam.
- Singe

note of credit: the music in this map is Spekkio's theme from the excellent SNES game Chrono Trigger. I'm not sure where the rain sound effect is from. :)