From a somewhat obscure blog here http://msdn.microsoft.com/en-us/library/aa717039.aspx, I found the source code and compiled it myself. As it turned out, this was included in a huge WCF and WF Samples download at http://www.microsoft.com/download/en/confirmation.aspx?id=21459. When it was unzipped, there were over 1200 files extracted! After that, went into folder: W:\Development\Downloads\Microsoft\WF_WCF_Samples\WCF\Setup\FindPrivateKey\CS, opened FindPrivateKey.sln and compiled it.

Then I went into the bin folder and run it like this :

DirectoryWhereFileIsLocated>FindPrivateKey My LocalMachine -t “af 50 4e f4 3b 57 ea f0 26 a8 b0 35 bf a7 0a a7 87 ef 10 5b” -a

And it returned:
C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\Ma
eKeys\3f67f438e6678b37604ae90622d1a568_3b18c4e6-fe0f-4826-b709-bc8b80bca037

Explanations of switches:

t=find by thumbprint

a=Outputs the absolute file name

Get examples of how to use FindPrivateKey from here http://msdn.microsoft.com/en-us/library/ms732026.aspx#1