If you go into your router can you see those devices and whichever device Media Center is loaded on? If no, you have a network problem. If yes, check these settings in MC 24:
1. Under Options > Media Network - make sure the top box is checked (Use Media Network to share this library)
2. Under Options > Media Network > Add or configure DLNA > Add "Generic DLNA" and then under Advanced Check DLNA and DLNAextra
3. Under Options > Media Network > Advanced check the three boxes: DLNA Server; DLNA Renderer; DLNA Controller
4. Under Options > Media Network > Client options check the boxes: Auto Sync; Play local file; Show playback zone
These are the settings on my system which work for two computers, a Sony Media Player, a Marantz receiver, and Panasonic TV. Whenever they are turned on, they just show up in Media Center as available under Playing Now.
Good luck