Search for these dll's and see if they are in the folder with Entlibconfig.exe. It looks like Microsoft.Practices.Unity.dll is the problem most of the time. At least it has been a problem for me in the past.
Microsoft.Practices.EnterpriseLibrary.Common.dll
Microsoft.Practices.EnterpriseLibrary.Configuration.Design.HostAdapterV5.dll
Microsoft.Practices.EnterpriseLibrary.Configuration.DesignTime.dll
Microsoft.Practices.Unity.dll