The simplest thing is to just download processing from processing then unzip it in your home folder eg /home/tux ignore the pacman version.
tar xzvf pro....tgz
You should read documentation:-
User centrality
Whereas many GNU/Linux distributions attempt to be more user-friendly , Arch Linux has always been, and shall always remain user-centric . The distribution is intended to fill the needs of those contributing to it, rather than trying to appeal to as many users as possible. It is targeted at the proficient GNU/Linux user, or anyone with a do-it-yourself attitude who is willing to read the documentation, and solve their own problems.