Thank you for reaching out to us on our forum. I am sorry to hear that you are having trouble connecting your slingbox using an HDMI cable. Unfortunatley the slingbox is not compatible with a HDMI cable because it is copy right protected, this would be why you do not get a picture using the HDMI cable. I found an article from our support site that explains it more in detail for you, I will post the link below.
Hope this helps,
The Sling Moderation Team