How to clone a private github repo

Which is the best way to clone a private repo,
I tested adding the the ssh key with ssh-add exec in the puppet recipe, but it doesn't works.

``` ruby
git::repo{'ux_app':
  path   => '/opt/app',
  source => 'git@github.com:edwin/ux_app.git'
}

```

how can I add the private key before clonning the repo.
thanks in advance.
