plug your external to your computer and type in terminal
type your password and hit enter
that will show you all the drives that are connected to your computer,your external will be something like
Or something like that, you can tell because it will show the file system type, if it is NTFS that will be your external for sure.
Once you know what device is your external you can mount it with
mount -t ntfs /dev/sdb1 /home/yourusername
then navigate to that directory
to see all the files on your drive.
Once you see your package there type.
dpkg -i yourpackagehere
good luck to you