You don't need to install anything from the Spring IDE nightly update site by yourself, please just add that site to the list of available update sites and use the Virgo milestone site to install the Virgo tooling. All the necessary Spring IDE parts will be installed from the nightly Spring IDE site automatically for you.
I guess the reason why you are not seeing anything when selecting the Spring IDE nightly site from the install new software dialog is that its components are not categorized, so when you deselect "group items by category" you will see all the bits and pieces from Spring IDE. But again, you don't need all that stuff and you don't need to install all that manually. Let Eclipse figure out for you which pieces it needs from Spring IDE.