The old monolithic clojure.contrib was available as a .jar from the same place you got the clojure .jar, and you used it by pointing your classpath at it. As far as I can tell, the new modular contribs aren't available in the clojure .jar -- instead, they exist as source files on github. What's the expected way for you to use them? Say, e.g., I wanted to use something in clojure.math.numeric-tower. What would I do?
I've found How do I install Clojure 1.3 with contribs on RHEL 6.1 / JDK7?, but the only answer ("use leiningen") isn't detailed enough for me to figure out. (Searching clojars for numeric-tower yields... nothing.)
You install a contrib module by adding its info to :dependencies in your project.clj file. The next time you run lein for something, it notices your change and automatically grabs the library for you.