You can cast to Chromecast yes. To do so you will need a mobile device. You cannot cast from your PC or Laptop. I have provided a link below that should be very helpful to you.
Hope this helps!!
Sling Media Support
A way to work around it is to launch your Google Chrome browser, then cast your desktop to Chromecast, then go to your Slingbox window and set it to full screen. It is not truly casting like a mobile device would do, and it is a little pixilated, but it works. If you can cast from a mobile device, it is a much better display.
BTW, if you have the original Chromecast, you may find that when you launch Slingbox on your mobile device, that you don't get the cast icon. I have worked out a workaround that works most of the time. They (whether it be Google or Slingbox) needs to fix their software, which, by the way, works fine with the 2nd Gen Chromecast. But....workaround for original Chromecast:
1. Launch Google Home
2. Cast screen
3. Start Slingbox
4. If the cast icon does not appear, close Slingbox and go back to step 3. This may take a few iterations. (Note, in some cases I had to end my screen cast first, but not always)
5. Once the cast icon appears, press icon to cast.