How to install a plugin?

Elasticsearch Version:0.19.9/0.19.20

I want to install plugin mobz-elasticsearch-head-ca2cccd.zip

Download it from https://github.com/Aconex/elasticsearch-head and put it into the local machine

Install it by the command ./plugin -url http://10.71.99.8:18080/mobz-elasticsearch-head-ca2cccd.zip -install elasticsearch-head

I find the directory has existed and the path is plugins/elasticsearch-head. But when I want access by url http://localhost:9200/ or http://localhost:9200/_plugin/elasticsearch-head/, I cannot get the right result

Can someone be so kind to help me solve this problem?