When using Xamarin Studio, if you receive error message XA5101 ("Missing NDK toolchains directory"), you will need to download Android NDK separately from:
http://developer.android.com/tools/sdk/ndk/index.html
Then it should unzip the downloaded file and open the Xamarin IDE and go to the following menu location:
Tools ==> Options ==> SDK Locations ==> Android
Select the location of the unzipped folder.
Then close the IDE and open it (force close / restart if it is saved in the task / process manager).
Alternative option:
Non-detection of NDKs may also be triggered due to inconsistent configuration for AVD Manager. More specifically, Xamarin can create devices and AVDs in your% USERPROFILE% .android, while emulating debugger / runtime EXPECTS AVD in the ADT directory. You may try to copy files and directories associated with ZAVD2 to% USERPROFILE% .android \ avd and see if this fixes the problem.
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[In case of an eclipse] I think the answer is here: fooobar.com/questions/186891 / ...
//goto Preference → Android → NDK and select the ndk location
// If your NDK location is already set, the constructor in the toolchain editor may be wrong. Go to project properties, C/C++ Build | Tool Chain Editor C/C++ Build | Tool Chain Editor and select Android Builder as the current builder.
// in your .bashsrc file, you may also need to add the env variable:
NDK_HOME=/opt/android-ndk-r8 export NDK_HOME
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