Can't stream audio to a wireless media player on the LAN

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I want to play my music files located on my Vista computer, via the wireless network link to an internet wireless media player. (Gear4 HouseParty Airwave)
The unit shows in the network map, yet I cannot find a way of telling Windows Media Player to stream to this device.
On media player Libray/More Options the tag for Sharing is greyed out, and a can't find a way to activate it. The Windows Help just goes round in circles, achieving nothing.
Can anybody help please?