Unable to resolve symbol when running code using Leiningen

from the leiningen repl you will have to switch to the namespace your function was defined in with the in-ns macro.

(in-ns 'myproject.core)

then the function should be available
you could also use that namespace from the repl to include it in the default (user) namespace.

(use 'myproject.core)

after that you may want to consider looking into the lein run, lein uberjar, and lein jar leiningen tasks.

In my projects, for a core.clj file which contains a namespace defined thus:

(ns my-project.core)

... I set the :main key in Leiningen's defproject map in project.clj:

(defproject my-project "1.0.0-SNAPSHOT"
  :description "My project description"
  :dependencies [[org.clojure/clojure "1.2.1"]]
  :main my-project.core)

So when I run lein repl, my core namespace is automatically loaded, and I see this:

mac:my-project scott$ lein repl
REPL started; server listening on localhost:31515.
my-project.core=>