'ADB install' works from the PC command prompt, not the ADB shell. You have the command prompt set to the PC folder where the apk is stored and then type 'ADB install app.apk'.
Edit: also you do not have to install apps that way anymore. Just flash my extras zip with my CWM and you can install apps directly in stock. No need to root.
Sent from my Nook HD+ running CM10.1 on emmc.
See my Nook Color Tips Thread here
and my Nook Color EMMC Partition Repair Thread here
and my Nook Color Updated CM SD Installation Instructions here
and my Nook HD/HD+ stock tips thread here
and my Nook HD/HD+ CWM Root Tools thread here