How to Install QGIS 2.6 plugins offline

"Due to various IT policies at my workplace, QGIS is installed on a machine (Window 7, 64bits and QGIS 2.6) that is not connected to the internet. I wish to install a couple of QGIS plugins on this system.

I have downloaded the required plugins and save them on usb key. How do I install them in QGIS?"

i 've already tried : -Unzip in folder \qgis\python\plugin =>says in plugin manager "plugins broken" displayed in red

As i'm a new user, I'll be very glad to read a step by step how to do that. NB: i'm seeking to make archeoCAD,numericalDigitize and numericalVertexEdit working.

Thanks to all of you.