To get messages from the message loader, use the command messages/get_messages:

svc=messages/get_messages&params={"indexFrom":<uint>,
				  "indexTo":<uint>,
				  "loadLocations":<bool>}

You can find an example of this request in the sample Messages.

[			/* array of messages */
	{
		...
	}
]

You can find message formats in the chapter Data format: Messages.