Native script [+name+] not found

I get an error :
ElasticSearchIllegalArgumentException[Native script
[similarity] not found]
I tried copying it to plugins/similarity folder in $ES_HOME. Also
tried installing using the 'plugins' command in elasticsearch 'bin'
folder. Even after adding the configuration to 'elasicsearch.yml' I
still get the same error. Am I missing something?

Can you gist the configuration you added to elasticsearch.yml? is
an excellent tutorial for native scripts on ES.