iphone 4 and HDMI cable to TV

I went to the Apple store and was advised by the staff there that in order to watch HBO Go, etc on my TV from my iphone 4, that I would need the HDMI Adaptor and the HDMI cable. I purchased both. When I hook the cable and adaptor up to the phone and TV, I can hear the audio, but the only thing showing on the screen is the HBO Go logo. Am i missing something?