Where is the Starcraft II custom map folder?
Actually, the way Blizzard has custom games working at the moment, it is not as simple as navigating to a folder to select the map. Many view this as a downfall in their current system, but this site is not the place for such arguments.
Rather, to answer your question, the only way for you to play this map is to either find it in the search engine for custom games online or to publish it yourself. Ideally, you can find it in the search engine online, but there are a limited number of maps that appear in that, so if the map you want to try is unpopular, your only option is to publish it yourself.
You have to open the map in the map editor and publish it just as you would any of your own maps. Unfortunately, this will create a copy on their server, and anyone found playing your copy will be counted differently from the original author's copy, which means that the stats for determining popularity will treat them as separate maps even though they are the same. It is very difficult for a new map to make its way up through the popularity list with this system.
This Team Liquid forum post goes into great detail about the shortcomings of the current custom map system and the workarounds for it.
The FAQ of the StarCraft 2 Map Editor (called "GalaxyEditor") gives some short information about the location and handling of custom maps:
How can I play maps i've previously downloaded? (Including the official maps)
Your downloaded maps can be found two different ways. The easiest of the two, is to go to File > Open > Battle.net Tab within GalaxyEdit. Otherwise, you need to search your Battle.net Cache for "*.s2ma" and then convert the .s2ma file to .SC2Map. However, the .s2ma filenames are just hashes of their true name, so good luck trying to find the map you need!
The Battle.net cache can be found at: C:\ProgramData\Blizzard Entertainment\Battle.net\Cache -- Win Vista/7 C:\Documents and Settings\All Users\Application Data\Blizzard Entertainment\battle.net\Cache\ -- Win XP
The folder containing Maps created by GE can be found: %USERPROFILE%\Documents\StarCraft II\Maps
http://us.battle.net/sc2/en/forum/topic/189279023