You use the ProntoEdit software and you must enable 'Developer Mode' if I recall correctly to allow you to edit the IR codes such that you could copy/paste Pronto Hex into a button.
Here is link to Daniel's utility to enable Dev Mode.[Link: files.remotecentral.com]
Once you have software installed, upload the existing configuration from your remote using the 'upload' feature. Then, paste the code data you need into MX450 editor.