1. mkdir /root/plesk
2. cd /root/plesk
3. wget http://download1.parallels.com/Plesk/PP11/11.0/CentOS5/parallels_installer_v3.12.0_build120601.16_os_CentOS_5_x86_64
this is 64bit CentOS version.
To give it execution permissions
4. chmod +x
parallels_installer_v3.12.0_build120601.16_os_CentOS_5_x86_64
5. Run the auto installer
./parallels_installer_v3.12.0_build120601.16_os_CentOS_5_x86_64
6. After installation, open Internet Browser and go to below URL:
https://your-server-ip-address:8443/