[Update: Microsoft starts to officially support Mono, so please simply use latest NuGet executable such as 3.5]
In part I and part II I have already mentioned the steps you need to follow. This post only described how to find out that Microsoft.Build.dll is missing.
On openSUSE (12.2) it is easy to find that xbuild and Microsoft.Build.Framework.dll is available under mono-devel package,
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lextm@linux-gtil:~> rpm -ql mono-devel | grep xbuild
Then it is obvious that Microsoft.Build.dll is not within this package,
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lextm@linux-gtil:~> rpm -ql mono-devel | grep Microsoft.Build*
For Ubuntu, the package structure is different. xbuild is in mono-xbuild, while Microsoft.Build.* are split into several packages such as mono-xbuild, lib-mono-microsoft-build-framework4.0-cil, and so on. I have to use which xbuild and dpkg -S /usr/bin/xbuild to locate the package name, which is not intuitive.