I dont have sample code handy, but there are a couple ways. One is to just poll the folder in a job, if you find a file, start the package. Or run the package and do the check up front. The Win API has a call that will monitor the folder and return when the folder changes, cleaner than polling but you have to have an app running to host the api call.
Andy
http://www.sqlservercentral.com/columnists/awarren/