The title is pretty much explanatory, isn't it?
You'll see, that's very easy, in your VM view, you'll see it instantly, just click on "Create Interface" button:
After clicking on "Create Interface", you'll have to fill the form, e.g selecting the network, MAC address and MTU:
And after clicking on "Create", you're done!
The new interface in now active on your VM. Enjoy Xen Orchestra!