Latest Mail Order is content plugin, not component!
Plugin is plg_content_mailorder-standalone-1.2-J25.zip.

Other plugin, plg_system_plginvoker-J25.zip is system plugin that allow you to include other plugins in many components. IN folder examples of plg_system_plginvoker is included examples for content, K2 and Virtuemart. 

Plugin "plg_system_plginvoker-J25.zip" is for Joomla 2.5 only and is needed only if you want to use MailOrder inside Joomla components that not parse content plugins!

1. Install plugin: "plg_content_mailorder-standalone-1.2-J25.zip".
2. Read FAQ for all other questions.

More info about this version is here: http://3dwebdesign.org/forum/revised-mailorder-content-plugin-for-j2-5-t1762