I'm not certain with xbox but with my ps3 I had an issue finding the picture out put when I initially set it up.
You'll need to make sure you have selected the correct hdmi channel on the tv and probably change the settings for video output on the xbox
(I had my ps3 set up via scart then changed it to hdmi from the scart channel as you need to see it to change the settings to hdmi and linear pcm etc)
If it's never been connected before surely it should automatically use the hd output?
Yik converter? Whats that?
Do you have a normal hdmi lead available to you?