To open a .ZIP file:
DOUBLE-CLICK the .ZIP file in a Windows Explorer (My Computer & navigate to file(s)) window, then from the window that pops up, drag the .map file(s) into the Halo\maps folder (nice to have open in another Explorer window) one at a time.
Then either delete the .zip file from which you just got the .map, or simply dimiss the popup window by selecting the "X" in the upper-right corner.
Only files ending in .map within your ...\Halo\maps folder matter. Anywhere else and you're just wasting disk space....