Adriana Cioica is correct about how to add the control. I have been able to use this existing code and add it to a silverlight 4 application. I have also updated the library a great deal since i last upload code onto this site. Not sure
if i want to release that piece of it because of maintanence and so forth. I have very little time to play with the library anymore.
The calendar you are looking for is the ClassicCalendar with the options of DisplayMode="Select" and SelectionMode="Multiple". You should be able to add a reference to your project of the "SLControlLibrary.Resources.Controls.Calendar.ClassicCalendar"
and use the "ucCalendar" object on your page. If the drag and drop does not work then you can add it manually by adding a reference to the assembly listed in your namespaces and create the object in xaml:
Just as it is done in the demo project in the SelectionCalendar.xaml file. To add the mult-select add another attribute to the ccDisplay SelectionMode="Multiple" and that should allow you to use the control.