It will be calling a repl of Leiningen. Rather than adding the dependencies to LightTable you could add them to your Leiningen Profile (~/.lein/profiles.clj)
The file would probably look something like this with your dependencies:
{:user {:dependencies [[math.combinatorics "x.x.x"]
[math.numeric-tower "x.x.x"]]}}
Generally this is not a very good idea. It will be a global thing and will probably cause you problems in the future. If you create an application you might find that these two libraries are available when they won't be for other people or on difference computers.
What would be a better option would be to create a new project using Leiningen. You can then edit your project.clj file to look something like this
(defproject math-thing "0.1.0-SNAPSHOT"
:description "FIXME: write description"
:dependencies [[org.clojure/clojure "1.6.0"]
[math.combinatorics "x.x.x"]
[math.numeric-tower "x.x.x"]])
Then when editing your clj file LightTable uses your project.clj file to start instrepl and will resolve any needed dependencies.