Here is a way I used to add a password to root so I could use it.
I entered, with out the quote marks of course, "sudo passwd root"
should put a password in for root which you can then use to su root.
I hope I am not wrong about this.

You can check this link
www.linuxnix.com/.../...o-root-user-in-ubuntu.html

I always found that the easiest way was to open a terminal, and type
"sudo xterm" - type in your standard password and bingo - it opens an
xterm as root (effectively).

I usually just open a terminal window, type 'sudo bash', then type my
password, the prompt that returns in the window is 'root@my-netbook'.
Give it a try. If it returns a root prompt for you, then do your install.

A word of caution..... be careful at the root prompt, it can do either
wonderful or dangerous things!!

Others have answered the actual question you asked, but I suggest it was the
wrong question. The normal way to accomplish what you wanted to do, is to enter
the command:
sudo ./install.sh

su requires you to have and know the root password.

sudo doesn't. It just needs the password for the administrative user
and then lets that person act as root, but without actually being root.
For instance, you still use your .bashrc fileand not root's .bashrc file.

So just use sudo and not su.

Or, do as people have said and give root a password.

ATTENTION: Word of caution before reading forward. It is potentially dangerous
logging in as root.

We all love utilising sudo / gksu for superuser powers, but it is possible to
permanently enable to root account so you can log into it.

Activate root user by assigning a password. Type the following:
$ sudo passwd root

Than type a new password that you will use to log into root, keep it long and
funky.

In the future you can switch to root user at the command prompt by typing su.
Click System | Administration | Login Window, than click Security tab and check
Allow Local System Administrator Login. Close program, log out and log back in
as 'root'.

AGAIN please note, running GUI as root is VERY DANGEROUS, but you cares, it is
your computer!