You need to prepare signing key file (sln) to sign those assemblies. Xamarin packages should be signed with its own key. To build *signed* assemblies, you need to run: make KEYFILE_SNK=/path/to/yourkey.snk SIGN_ASSEMBLY=true SIGN_ASSEMBLY_SPEC=SIGNED Xamarin will publish "signed" version of those assemblies on the component store, so that Starter users can make use of Rx assemblies. If you don't need any signing, simply build Rx_Xamarin/android/Rx_Xamarin_android.sln.