Download and install the Blend Preview for Silverlight 5 . Then update your links.
C: \ Program Files (x86) \ Microsoft SDK \ Expression \ Blend \ Silverlight \ v5.0 \ Libraries \ Microsoft.Expression.Interactions.dll
C: \ Program Files (x86) \ Microsoft SDK \ Expression \ Blend \ Silverlight \ v5.0 \ Libraries \ System.Windows.Interactivity.dll
(Note: although this is a preview version, it is fully functional, as far as I can tell. The only limitation is that it can only be used for Silverlight 5.)
Update: From EULA:
I. The right to use and distribute.
The following codes and text files are listed: Distributable Code.
· REDIST.TXT files. You can copy and distribute the object code specified in the REDIST.TXT files.
redist.en.txt:
Expression Blend SDK for Silverlight 4
==================================================== ===
The following list is a list of files available using the Microsoft Expression Blend SDK for Silverlight 4 software for redistribution in the Expression Blend SDK for the Silverlight 4 license.
Under the terms of the software license, you can distribute files contained in the following directories, unmodified as part of your programs:
Library
Patterns
{Program Files} \ MSBuild \ Microsoft \ Expression \ Blend \ Silverlight \ v4.0 \
Update 2: Redist.en.txt (C: \ Program Files (x86) \ Microsoft SDK \ Expression \ Blend \ Silverlight \ v5.0) says:
Preview Expression Blender SDK for Silverlight 5
==================================================== ===
The following list is a list of files available using the Microsoft Expression Blend SDK Preview for Silverlight 5 software for redistribution in the Expression Blend Preview view for the Silverlight 5 license.
Under the terms of the software license, you can distribute files contained in the following directories, unmodified as part of your programs:
Library
Patterns
{Program Files} \ MSBuild \ Microsoft \ Expression \ Blend \ Silverlight \ v5.0 \