Selecting a network DLNA source in VLC is a little obscure, but I'm sure it has been documented on here somewhere. I doubt Windows Media Player has the right codecs, or that it is able to negotiate the DLNA media navigation. Google "site:hummy.tv/forum vlc", or check my index (link in my sig panel). The Humax DLNA server must be turned on (Menu >> Settings >> System >> Internet Setting >> Content Share = On), and given time to index the content when first turned on. Up to date versions of VLC are capable of playing HiDef content, but the Humax will not serve HiDef unless you unlock the content using the custom firmware auto-unprotect package. No such restrictions with StDef, and no need to decrypt content first.
As far as accessing PC content is concerned, a DLNA server has to be active (I have no experience of Media Tomb; I made XBMC work at one time although others have failed with it) and you have to register the content (not just drop stuff into a folder). The Humax DLNA client will only list media of a specific type at any one time - select video, music, or photos from Media >> Media (yellow), it defaults to video so no MP3s will be listed until you select music.