API Plugin not found / Code 400

Hi,

I want to use your api to connect to easysocial. I installed the newest com_api in joomla + the plugin called "users" but when I try to call it I get the following response:

SUCCESS: {
code = 400;
message = "Users API Plugin not found, Users is probably not installed";
}

this is the request url I used: http://localhost:8888/socialapp/index.php?option=com_api&app=users&resource=users&key=ba0fda310928525125b318651add4fd5&format=raw

where is my mistake?

Thanks a lot!