I can't find Remote Desktop on my OSX 10.11

Perhaps you're getting confused? Unless you've purchased the App from the App Store you won't have it and therefore you won't find it. What is built into the OS is the ARDAgent, the client part the full-blown App that ARD 'taps' into.

Alternatively something may have gone amiss with the ARDAgent itself (perhaps you've been fiddling with the OS?) and hence the reason why you're asking the question? To see the if the ARDAgent is installed launch the Terminal App - you'll find the App in the Utilities folder from the Go Menu - and issue this command:

system_profiler SPApplicationsDataType | grep ARDAgent

You can copy and paste the above into the Terminal window. Be patient as it can take a while for the command to run. If successful you should see something like this:

/System/Library/CoreServices/RemoteManagement/ARDAgent.app

Which means you have the client part of ARD. Once you've verified this and you haven't actually purchased the App as described here:

http://www.apple.com/uk/remotedesktop/

https://itunes.apple.com/gb/app/apple-remote-desktop/id409907375?mt=12

Then simply launch the App Store on your Mac, search for the App and purchase it.