Newbie to Linux question!

I've downloaded and installed CentOS 5.4 on a 'PC'

I have downloaded OTRS-2.4.7.tar.gz to /tmp

I am logged in as root (well default admin user that I set up)
I open Applications>Accessories>Terminal
It has the prompt admin@localhost
I cd to /opt then run the command 'tar xf /tmp/otrs-2.4.7.tar.gz

I get loads of Cannot open: No such file or directory

What am I doing wrong?
Should I login as root for this, then admin for the OTRS install

Thanks (be gentle to a Linux newbie)

Neil