Following are the three step plesk installation in your Ubuntu Server.
Step 1 : Plesk Debian repository should
be added to the sources list
Navigate to the URL http://autoinstall.plesk.com
Open it in a web browser to see which Plesk versions are available.
Then add the repository record for your desired Plesk version into
"/etc/apt/sources.list." For example, you would use the
below line for Plesk 11.0.9 :
deb http://autoinstall.plesk.com/PSA_11.0.9/ etch all
Step 2 : Update
the cache packages list:
apt-get update
Step 3 : Install
Plesk from command line. The following will install the plesk package
apt-get install psa