First, we recommend you to check the following links in order to determine if the IR Blasters are working in a proper way.
If you follow the steps and the IR Blasters are working fine but the issue persists, we encourage to check the thread provided by alanrichey42
The button is disabled for me as well. The whole Safari & Slingbox experience is a nightmare, Safari plug in is either poorly written or was never properly tested because each time it tries to download something (firmware for Slingbox or remote control bin file) Safari crashes. For Slingbox setup I had to use old Windows machine but I'm unable to see remote control in Safari at all. Only way how you can to switch channels is to use TV Listings in a separate popup window.