Does this control work with SL5? I tried adding a NuGet reference and tried adding a simple PopupMenu and get the following error:

The tag 'PopupMenuItem' does not exist in XML namespace 'clr-namespace:SL4PopupMenu;assembly=SL4PopupMenu'.

I suggest you recompile the source code for the menu after updating the SL4 references in the project to SL5. This should get rid of the exception.