This is the same exact problem that I've been having. I've rebooted (and also unplugged/ replugged) both the Slingbox and the Roku, with no success. I can watch tv on my iPhone 6, and I can see another Roku we have in our house, but I am not able to see the Roku in my home office. I'm also not able to 'discover' it by adding the IP address (which is less than the 11 character maximum). This has (generally) worked just fine for me for the past year plus, but now I'm completely unable to see the Roku as an option.
It would be really great if someone from Slingbox would provide some additional assistance (beyond what exists in the current Help functionality...because I've already tried that with no success).