Unless the HUB is different than other models, there is no way to put an audio input directly into the WDTVs. There are better ways to play your music from an iPod or iTunes.
To play music from your iPod there are connector wires made for doing so into your music system amp/receiver or even TV.
With the HUB (or other WDTVs), you can access the iTunes music folder on the PC and stream them from there with the HUB to TV. Or, you can copy the iTunes music files, all or some, to the HUB's hard drive for playing. Or, do it either way, separately.
Locate the complete user manual in the WS Support for you model WDTV for all the how-to's.