You don't have to open the extension pack to use it. Just go to preferences in VBox, go to the extensions page and tell it to load the pack. It will then automatically install.
Hope this is helpful,
MCP
Thanks MCP but -- I think I'm being a bit dumb here -- it wants an
administrative password. Where do I get this? I've tried a few including just a return.
That is your master password. The one you created when you installed.
MCP