Unable to find RestClient to send MMS, Where is this located?

You can find this on Nuget and GitHub:

Nuget: https://www.nuget.org/packages/RingCentral.Client/
GitHub: https://github.com/ringcentral/ringcentral-csharp-client

The MMS Demo is here:

https://github.com/ringcentral/ringcentral-csharp-client/blob/master/RingCentral.Test/MMSTest.cs

You can also find links to this on our Developer Portal with the rest of our SDKs:

https://developer.ringcentral.com/library/sdks.html