Where is the .NET Framework 4.5 directory?

.NET 4.5 is an in place replacement for 4.0 - you will find the assemblies in the 4.0 directory.

See the blogs by Rick Strahl and Scott Hanselman on this topic.

You can also find the specific versions in:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework

EDIT: This answer was correct until mid-2013, but you may have a more recent version since the big msbuild change. See the answer from Jonny Leeds for more details.

The version under C:\Windows\Microsoft.NET\Framework\v4.0.30319 actually is .NET 4.5. It's a little odd, but certainly mscorlib there contains AsyncTaskMethodBuilder etc which are used for async.

.NET 4.5 effectively overwrites .NET 4.