Put the four relevant lines in your ~/.vimrc
file. The plugin has to be installed separately from here.
let g:repl_cmd = '/home/pi/APL/svn/trunk/src/apl --noSV --rawCIN --noColor'
let g:repl_stop = ')OFF'
let g:repl_send = "'REPL-VIM'"
let g:repl_detect = 'REPL-VIM'