For the life of me, I could not find the interaction assembly from Microsoft. And “Googling” the matter only reveals that I was most certainly not the only one with that problem! I learned, that it is included as part of the Expression Blend SDK for Silverlight 4.
And can be found here (on a Win8 machine):
C:\Program Files (x86)\Microsoft SDKs\Expression\Blend\Silverlight\v4.0\Libraries