It's a bit confusing. let me try to clarify thing.
Plugins compiled with version <R20 are not compatible with R20. You have to recompile them.
Plugins compiled with version R20 are not compatible with R21. You have to recompile them.
Plugins are not backward compatible (plugins compiled with version r20.059 will not work with version 20.012)
For each version, you should refer to this page to know witch version of Xcode and Visual Studio to use
In the zip you provided, you are using the framework 20042 so it will not work with version R21.
You are talking about R20.026 (i don't know if you was referring to R21.026 witch is the actual version)
You have to unzip the SDK.zip found in the cinema4D directory somewhere in your hardrive where you have write permission. Than use the projectool to generate the project files.
If you follow those directives, you should be good to go.
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