On OS X double click on the dmg file and follow the instructions:
Once installed, go to preferences and ensure the default machine location is a valid location on your PC or Mac
Check the Host-only Network Configuration
If you already had VirtualBox installed, check that you have a Host-only network configured on the host. If this is a fresh install of VirtualBox, check that the Host-only network adapter is configured as below.
Use the Oracle VM VirtualBox Manager GUI, Select File --> Preferences, then in the VirtualBox - Settings window, select Network. In the Network pane, select the Host-only Networks tab and check you have an entry named vboxnet0. If not, click the add host-only network button to add it.