It depends on the version of PresentationFramework you want when you need to add it as a reference. Basically, you will find it in :

\Program File\Reference Assemblies\Microsoft\Framework

(for 64-bit compilation, or 32-bit on a 32-bit OS)

or

\Program File (x86)\Reference Assemblies\Microsoft\Framework

(for 32-bit compilation or 32-bit on a 64-bit OS)

These are only the root folders. From here, you can go for example in "v3.0" or ".NETFramework\v4.0" for example.

So you just need to add a reference to a full "[path]\PresentationFramework.dll" instead of just "PresentationFramework.dll", for example:

"C:\Program File\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\PresentationFramework.dll".