Alright! I'm going to assume you know the basics of how to use the terminal.

Open up x-term and use the command "su".

It should prompt for the root-password. Enter the password and you'll (hopefully) be root.

Cd to where you saved your rootsh-package and type;

dpkg -i rootsh_1.5_armel.deb

And voila, that should install rootsh on your device.

[EDIT]

Come to think off it, as long as you are root you can just use apt for installing rootsh. Assuming you have extras in your repos, that is.

If you encounter any errors with the .deb instead issue the following command;

apt-get install rootsh