This is first question that ran though my mind once I finished installation of SilverLight Toolkit today. When we install the toolkit, the installation wizard does not ask us for any installation folder options and after completion of installation there will not be any entries in to the All Programs section of start menu.
After going through the documents, I found that installer silently places all the binaries, themes, samples documents under program files folder depending on the version of the toolkit. If you installed version 4.0 of the toolkit then it will be placed in the folder
C:\Program Files\Microsoft SDKs\Silverlight\v4.0
Here is the list of other useful folder of SilverLight toolkit that we refer to often
Bin
C:\Program Files\Microsoft SDKs\Silverlight\v4.0\Toolkit\Apr10\Bin
Samples
C:\Program Files\Microsoft SDKs\Silverlight\v4.0\Toolkit\Apr10\Samples
Themes
C:\Program Files\Microsoft SDKs\Silverlight\v4.0\Toolkit\Apr10\Themes
Source
C:\Program Files\Microsoft SDKs\Silverlight\v4.0\Toolkit\Apr10\Source
Please note this above listed folder names will not be exactly same on your computer as they vary from one version to another. First open the base folder C:\Program Files\Microsoft SDKs\Silverlight and then navigate through the available folders for locating the required ones.
Hope this helps you.