安装的时候断了一下网，然后它告诉我可以这样手动安装(由于下文在 web 上找不到，所以全文转载了)：
适用于 2016-11-17 CST 发布的版本
It was not possible to complete an automatic installation. This might be due to a problem with your network, proxy servers or an unsolvable installation conflict. At this point, you can continue the installation by manually downloading and installing the independent pieces. The list below shows each component that you need to install.
Mono Framework
Download the Mono Framework v4.8.0 archive to any directory on your disk, double-click it and follow the instructions on screen. Please accept all the defaults during the installation, including the target drive.
Android SDK
Download the Android SDK for Mac OS X version 24.4.1, open the downloaded archive and copy the contents of the folder found at its root directory (most probably named android-sdk-macosx) to the ~/Library/Developer/Xamarin/android-sdk-macosx directory.
Run ~/Library/Developer/Xamarin/android-sdk-macosx/tools/android in order to start the Android SDK Manager application. In the GUI make sure the following components are installed or selected for installation:
- Android SDK Tools
- Android SDK Platform-tools
- Android SDK Build-tools
- Android API 23
Once all the required components are selected (you can select others as well, if you wish) start the installation.
Download the Android NDK for Mac OS X version r10e, open the downloaded archive and copy the contents of its root directory (most probably named android-ndk-r10e) to the ~/Library/Developer/Xamarin/android-ndk directory.
When the Android SDK installation is finished you might want to start the "Android Emulator Manager"(~/Library/Developer/Xamarin/android-sdk-macosx/tools/android) and create as many emulator images as you might need for your development. This step can be performed at any time, on an as-needed basis.
Visual Studio for Mac Preview
Download the Visual Studio for Mac Preview v7.0.0 archive to any directory on your disk, double-click it and then drag and drop the Visual Studio for Mac Preview icon to the Applications folder.
Xamarin.Android
Download the Xamarin.Android v7.1.0 archive to any directory on your disk, double-click it and follow the instructions on screen. Please accept all the defaults during the installation, including the target drive.
Xamarin.iOS
Download the Xamarin.iOS v10.4.0 archive to any directory on your disk, double-click it and follow the instructions on screen. Please accept all the defaults during the installation, including the target drive.
Xamarin.Mac
Download the Xamarin.Mac v3.0.0 archive to any directory on your disk, double-click it and follow the instructions on screen. Please accept all the defaults during the installation, including the target drive.
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