Yup, sorry I didn't provide those information. When 2.1 is near shipping, we'll make sure to update our documentation on docs.nuget.org.
Here are enough details to get you going though.
To set a target framework for PCL, name your folder in this format:
portable-<list of supported frameworks separated by + sign>
portable-sl4+net45+wp7 : this supports Silverlight4, .NET 4.5 and Windows Phone 7
(Note that in the current build, we haven't supported Xbox yet. We are considering adding it.)
For your first scenario, you're right. The package will not be installed into the project.
For the second scenario, you can include both versions in one package. Just use the folder format as described above, one with Xbox and one without. However, as I said, NuGet doesn't recognize the Xbox moniker yet, so the folder will be treated as invalid.