whimrepl is not the default, to use it you will have to add this line to your .vimrc
:
let g:slime_target = "whimrepl"
When you invoke vim-slime
for the first time, you will be prompted for more configuration.
whimrepl server name
This is the name of the whimrepl server that you wish to target. whimrepl
displays that name in its banner every time you start up an instance of
whimrepl.