This issue has come up several times: You want to install StreamInsight from the MSDN image, but there is no key pre-entered during setup.
Microsoft StreamInsight ships as part of Microsoft SQL Server 2008 R2 and does not require a separate product key. The same product key (PID) is share with Microsoft StreamInsight. The ISO for MSDN subscriptions is pre-PIDded and does not require the user to enter the product key when installing SQL Server 2008 R2. However, this PID is not automatically entered for StreamInsight setup. The PID can be found in X:\x64\DefaultSetup.ini or X:\x86\DefaultSetup.ini (assume you have the ISO mapped to drive X:).