I noticed that my past two guests (Oracle VM and Windows 7 VM) were both unable to connect to the Internet once I had them installed. Here are the steps I took to correct the problem on OpenXenCenter and get Internet connectivity working on my XCP guests.
1) For your specific VM – select the Network Tab
2) Click the Add Interface button
3) A new Window appears – New Virtual Interface Network
a) Pull down the arrow on the Network: Location item and select Network 0
b) MAC Address – I left this on Auto-generate
c) Limit – I left this blank
d) Select OK
4) Shutdown and Restart your VM and the Network Tab will now show you a MAC address with the Active column set to TRUE
That was all it took to setup a Network Interface from the Guest to the Domain0 Guest.