help with nexus 7" tab

Hello guys,

This is the first time ever that I use nexus tab.. in the past I used to copy the APK file to a directory in the device and install it from there (by clicking on it)..
Today I was able to copy the file as usual, but when tried to install it I couldn't because it's name was grayed out and disabled.
Any information appreciated.

Solved!
Nexus 7 has no file manager, so I downloaded ES File Explorer File Manager from the store and now working perfectly.