Hello,
The reason the error occurs is that the names have changed in 3.x so for example: "ISessionCOMManager" has changed to "ICOMSessionManager".
There is a more up to date "SpokesSDKCOMNETSample" that you could try since the sample in the link you have provided is no longer supported (I will shortly update that link to point to the latest sample). To get to this sample, please download the latest SDK (currently 3.6) and go to "C:\Program Files (x86)\Plantronics\Spokes3G SDK\Samples" where you will find a zip file containing all the samples for the SDK. The sample you are looking for is in "Spokes3GSDKSamples".
Let me know if this has helped or if you have any more questions.
Regards,
Sandra