According to the community documentation about using the terminal,
sudo: Executing Commands with Elevated Privileges
- Most of the following commands will need to be prefaced with the sudo command. This elevates privileges to the root-user administrative level temporarily, which is necessary when working with directories or files not owned by your user account. When using sudo you will be prompted for your password. Only users with sudo (administrative) privileges will be able to use this command. You should never use normal sudo to start graphical applications as Root (Please see RootSudo for more information on using sudo correctly.)
So, because apt-get installs software and thus affects the system, you need to use the
sudo command to give yourself administrator privilages.
Thus, you command should be
sudo apt-get install myunity
If you want to update your system, run
sudo apt-get update
sudo apt-get dist-upgrade
This will update your system's package database and then install any upgrades.