In this case，试试这个方法行不行
To build for Xamarin.iOS, you’ll need:
the latest iOS SDK.
the latest version of Xcode.
Mac OS X Yosemite(10.10) & above.
The latest version of Xcode can be downladed from the iOS Developer Center (login required), or via the Mac App Store:
Downloading the Xamarin Unified Installer
The first step in obtaining Xamarin is to download the Xamarin unified installer, which is found at http://xamarin.com/download:
Fill in your details, as required, and click the Download Xamarin for OS X button.
Running the Xamarin Installer
Once the installer is downloaded, run it by double-clicking the Xamarin logo, illustrated below:
You may be prompted with an alert dialog, similar to the image below. In this case, click Open:
Step 1 – Xamarin License
The first step in the installation requires you to review and accept the license in order to proceed. To do this, click the checkbox and press Continue:
Step 2 – Identify and Select the Required Components
Next, the installer will inspect the system to determine which required components are missing and need to be downloaded and installed. You can select which products you wish to download here:
The Xamarin installer should automatically inherit network proxy settings for the active network interface from the system. To manually configure the proxy open Application > System Preferences.app and select the Network icon. Then select the appropriate network interface and click the Advanced button. Finally, set the configuration values in the Proxies tab:
Step 3 - Install the Components
After identifying missing components, the Xamarin installer will download and execute the installers for the platform dependencies, as shown in the screenshots below.
The Android SDK installation location can be configured here, if you choose to also install Xamarin.Android at this time:
The installer will then show the items to be downloaded and installed; click on Continue to start the this process:
The Xamarin unified installer will start the download and install process of the selected items:
Be aware that during this part of the installation, you may be prompted to elevate the permissions necessary for individual components that are needed to complete installation.
Step 4 - Complete the Installation
To begin developing a Xamarin.iOS application targeting the iOS simulator, simply click the Launch Xamarin Studio button.
Step 5 - Activation
If this is your first time using Xamarin, and you do not have a Xamarin account you can sign up for one here.
If you have an existing Xamarin license do the following the activate it.
Click Log In on the Welcome Screen, and enter your Xamarin credentials:
Upon successful activation, Xamarin Studio will notify you that the activation has been completed:
Starting a Free Trial
If you are running Community Edition, and wish to try out the Enterprise features, you can avail of the free 30 day trial.
This process is explained in more detail in the Beginning a Xamarin Trial guide.
That’s it! Everything should be good to go. Learn about Device Provisioning, or head on over to the Hello, iPhone tutorial to build your first iOS application!