According to http://msdn.microsoft.com/en-us/library/bb552364.aspx
You can find the WCF Test Client (WcfTestClient.exe) in the following location: C:\Program Files\Microsoft Visual Studio 9.0\Common7\IDE\
If you have Visual Studio 2010 installed on a 64bit OS, it is found here:
C:\Program Files (x86)\Microsoft Visual Studio 10.0\Common7\IDE\WcfTestClient.exe