Have you bought it (or downloaded the limited free version), and installed it?
If you haven't, then you definitely can't use it until you do...
http://www.microsoft.com/expression/products/encoderpro_overview.aspx[
^]
Even then, from the limited reading I have been doing, I suspect you are on a loser - Microsoft Expression Encoder is an application, rather than a set of .NET classes for you to include in you application...