note: look at line 6 and line 32

core.cljs

(ns myproject.core
    (:require [reagent.core :as reagent :refer [atom]]
              [reagent.session :as session]
              [secretary.core :as secretary :include-macros true]
              [accountant.core :as accountant]
              [myproject.core.playground :refer [play]]));; i added this line

;; -------------------------
;; Views

(defn home-page []
  [:div [:h2 "Welcome to myproject"]
   [:div [:a {:href "/about"} "go to about page"]]])

(defn about-page []
  [:div [:h2 "About myproject"]
   [:div [:a {:href "/"} "go to the home page Lewis"]]])

(defn current-page []
  [:div [(session/get :current-page)]])

;; -------------------------
;; Routes

(secretary/defroute "/" []
  (session/put! :current-page #'home-page))

(secretary/defroute "/about" []
  (session/put! :current-page #'about-page))

(secretary/defroute "/playground" []
  (session/put! :current-page #'play)) ;;


;; -------------------------
;; Initialize app

(defn mount-root []
  (reagent/render [current-page] (.getElementById js/document "app")))

(defn init! []
  (accountant/configure-navigation!
    {:nav-handler
     (fn [path]
       (secretary/dispatch! path))
     :path-exists?
     (fn [path]
       (secretary/locate-route path))})
  (accountant/dispatch-current!)
  (mount-root))

playground.cljs

(ns myproject.core.playground)

;;playground 
(defn play []
  [:div [:h2 "Welcome to my playground"]])