Here is what I did..
I tried using Fire Fox browser and did this:
We do have a work around that you can try if you like. This is not a solution but should allow you to open the plug-in.
For Firefox 43.0
In the address bar type: "about:config"
A message will pop up for dangers of the these options agree to them if you would like to continue.
Search for "xpinstall.signatures.required". It should have a value of True.
Right click on "xpinstall.signatures.required"
then click on "Toggle" the value will change to false. After that just restart Firefox and try watching your Slingbox
I did this and was able to download the new plug-in and start watching...I still have not had any luck on Explorer or Chrome browers.
Hope it helps