It looks like the problem is happening because Reflector now runs as a 64-bit app on 64-bit systems and this screws over the Silverlight discovery. We have a fix for this in 7.x which we'll backport into 6.8 and then put out as a 6.8.2 release, which you'll be able to get via Help > Check for Updates in Reflector. It'll probably be a week or two before this fix is available.
In the meantime, as a workaround, you might try forcing it to run as 32-bit using the CorFlags utility included with the .NET Framework SDK:
CorFlags.exe reflector.exe /32Bit+ /force
Output will be something like the following:
Microsoft (R) .NET Framework CorFlags Conversion Tool. Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.
corflags : warning CF011 : The specified file is strong name signed. Using /Force will invalidate the signature of this image and will require the assembly to be resigned.
Hope that helps.