Installation should be easy:  just drag Vim.app to where you want to keep it.  I put it in /Applications/ . Unless you want a maximally vi-compatible version of vim, it is recommended that you copy the standard startup files to your home directory, and name them .vimrc and .gvimrc .  From a shell, 
$ cd Vim.app/Contents/Resources/vim/runtime
$ cp vimrc_example.vim ~/.vimrc
$ cp gvimrc_example.vim ~/.gvimrc
 

So.. that means that everything in /Applications/ is some kind of archive/directory. Interesting.