Note that Pear's Logging package is a lot more flexible, so I recommend you use that instead of CakePHP's built-in logging.
Use this code to add a debug message to the CakePHP debug log:
$this->log("Upload action accessed", LOG_DEBUG);
Note that using something other than LOG_DEBUG will log the message as an error.
Run the code once and you should be able to see the message in $CAKE_APP/tmp/logs/debug.log.