Lets first do the setup in one line. Get my vim script downloader.
Use the script to add the script with script ID : :1879 (vim-autocomplpop) to your list of plugins for vim and you are done.
If you are using snipmate, then make sure that you read the description of the plugin to add the required lines to the vimrc file to get the full functionality. You would love to type henceforth in your favourite editor.