By default, Sky+ HD does not have ITV HD avaialble in its list of channels so you have to add it manually.
Go into Settings > Sky+ Setup > Add Channels
Enter the channel details as below:
Frequency (GHz): 11.427
Symbol Rate (Mbaud): 27.5
Then press the Yellow button to search for channels.
It will do a scan and then display a list of channels it has found.
ITV1HD is named 10510
Highlight it and then choose store channels. when you want to watch it you’ll need to go into the Other Channels section and press select.
You can repeat the process for the other channels it finds in the list when searching such as Freesat etc.