It seems that you are having some issues with your Slingbox TUNER since you cannot get the connection to your DirectTV box.
The main issue is that the Slingbox TUNER (a legacy Slingbox model) will work only if you feed it with an analog TV signal. Since you have upgraded to the DirectTV signal (digital), the Slingbox is not able to stream the digital signal.
Unless you get a latest Slingbox hardware version (such as Slingbox SOLO or the Slingbox PRO-HD), your only option is to use a Digital to Analog Converter. However, you might have some issues in order to control this device with the Slingbox since it might not be included on the supported devices list.