To connect a virtual server to a network, you first need to have an available organisation network. Each new service will include a preconfigured network, but please see the Ogranisation Networks article should you need to setup additional networks.
When deploying a vApp, you can select to connect to a network as soon as the vApp completes deployment. Please see this article for deployment instructions.
After a server has been deployed, you can change the network it has been connected to, or connect it to a new network by selecting either vApps, the desired vApp, then click the label of the virtual server, or select Virtual Servers and click the label of the desired virtual server. Once in your server's overview screen, find 'Networking' at the top, then select Network Interfaces.
Click the 'New Network Interface' button, then select the desired organisation network from the dropdown.
Once submit is clicked, the server will be attached to the network. You may need to restart your server before this takes effect.