You are using a really outdated tutorial. According to Leiningen docs, crossovers are deprecated. Moreover, you only need them if you intend to share code between backend/frontend, which I doubt you do.
The simplest starting point for a ClojureScript project that I know of is:
$ lein new mies webviz && cd webviz
$ lein cljsbuild once
$ open index.html
Which will present you a blank page, and a "Hello world!" in javascript console.