Get Homebrew installed on your mac if you don't already have it
Install highlight. "brew install highlight". (This brings down Lua and Boost as well)
Copy your source code to the clipboard
Highlight your source. "pbpaste | highlight -O rtf | pbcopy". This will highlight the source code in RTF format and copy the result of that operation to your Mac's clipboard.
Paste into your slide in Keynote. Keynote will create a new text box with your highlighted code. You can adjust the width as needed, and adjust the font size with cmd+ and cmd-
Create your highlighted text using a specific font, font size, (language)[http://www.andre-simon.de/doku/highlight/en/langs.php] and (style)[http://www.andre-simon.de/doku/highlight/en/theme-samples.php].
pbpaste | highlight --syntax=java --font-size 16 --font=menlo --style=solarized-dark -O rtf | pbcopy