I have Xamarin installed on the Visual Studio 2013 on Windows Operating System. It works fine.
There is Xamarin tools in VS2013 as follows:
My problem
It is not required to delete previous versions of VS. I have Xamarin Studio, VS 2013 and now VS 2015 and working perfectly. I moved to VS 2015 due to recent purchase of Xamarin by MS and that VS 2015 has Xamarin included.
I believe the issue with this question is was that your VS 2015 install did not include the add ins as pointed out in another answer in this question.
After the VS 2015 install -the only caveat was projects created initially in Xaramin Studio and then Opened in VS 2015 to needed to have the build configuration in the project tweaked. If not you could not get the project to build and then be deployed to an emulator or device for debug testing.
I had some problems with my emulator best way to fix the problems is to uninstall everything (emulators, android sdk, vs, xamarin) and reinstall it.
It is also important to delete other or older versions of visual studio. Then you should probably see you options.
Before any install or uninstall actions. Try open some sample Xamarin project, in most cases during opening project in VS 15, it will show you, what needs to be installed or what is missing / corrupted.
Xamarin samples
Visual Studio 2015 is little bit different from 2013 version, I am also using xamarin on visual studio 2015. But that is not listed under tools menu like 2013.
Maybe developers had changed that, you can go to "start a new project"(just like you start new app development), there you can find xamarin/android projects list, there you can start new android projects.
Here is the solution:
I have deleted all the existing versions of visual studio ( 2010, 2012, 2013) except Visual Studio 2015 and then installed the Xamarin, and then I could able to see Xamarin on Visual Studio 2015.
I have installed Xamarin in my Visual Studio 2015 two days ago and it showing Xamarin under Tools. Below I have described the approach which I have followed to install Xamarin in Visual Studio 2015.
Go to Contol Panel >> Program Features >> Visual Studio 2015 >> Change>> Modify
1. Select C#/.Net(Xamarin) under Cross Platform Mobile Development
2. Check if these three red marked items are checked or not.
3. And then press update.It will download all the required packages to run Xamarin on VS 2015.Hope it helps you!!