There are two options:

a) The MSFT Way:

Alternatively MSFT says that starting the command

Reset the Visual Studio 2010 experimental instance" (you'll find this command in the start menu | All programs | Microsoft Visual Studio 2010 SDK)

should work. And i think that way it should work with VS2008 also.

For me it did not always work so i came up with one more way to do it.

b) The tangible way:

In order to enable T4 Editor V1.9 in the experimental hive you have to copy over files and folders from the following folder:

C:\Users\<user>\AppData\Local\Microsoft\VisualStudio\10.0\Extensions

to  C:\Users\timfis\AppData\Local\Microsoft\VisualStudio\10.0Exp\Extensions

Then start VS Experimental Instance goto Extension Manager and enable the new extensions.