Enterprise Library 3.1 was published for Visual Studio 2005. That is to say: the Guidance Packages and the integrated Configuration Tool were built to work with Visual Studio 2005. The rest of the library works against .NET 2.0 and continues to work with .NET 3.5 apps.
We have done some work to get the integrated tool to work in Visual Studio 2008. If you run the following registry script it will change the keys where VS looks to load the integrated tool package. After you run the script, you will need to run devenv /setup from the Visual Studio 2008 command prompt.
You can download the file from the EntLibContrib project on CodePlex from here.
I will try and be a little clearer for the steps:
1. Install EL 3.1 (you don't need the Guidance Packages)
2. Run the script
3. Run devenv /setup from the VS 2008 command prompt
Wheels from the album "Pressure Chief" by Cake