No idea about 1 and 2.
Try to download it again from this thread
. Looks like there was something wrong with the file.
Also you need the Library/Application Support/ folder in your home directory, not the one in root.
Once you copied the pk3 into the correct place, open the console at JOs menu screen (I do it by pressing '>', but I have a german keyboard, don't know if that makes a difference) and type "map imphoth_a".
Hope this helps. ^^