Here is short guide for installing app via terminal in Nitdroid. as i have installed nitdroid today and my gprs connection doesn't work with nitdroid here is the guide for install any app via terminal.
place your apk file in MyDocs.
boot nitdroid then go to menu>dev tools>terminal emulator.

rename your apk file to short and simple name.e.g i wanted to install astro file manager for i just renamed it to astro.apk after opening terminal type:

su (it will request for super user permission grant it)
cd /sdcard
install astro.apk /system/app/astro.apk

it will install your app quickly. it will not write anything in terminal. just check your menu.

Good luck.