Hey Roy Elder!!
Thanks for contacting us here on the forums!! Unfortunately, the way you are trying to cast over to the Chromecast is not a supported method. It would require a mobile device (iPad, iPhone, Android tablet or Android phone) with the Slingplayer application installed on it. From there, you would need to ensure the mobile device and the Chromecast are on the same network and utilizing the casting icon you would cast over to the Chromecast on the TV. I have included a support article that will give you some information on using the Chromecast in conjunction with your Slingbox:
Hope this helps!!!