running mint 9 isodora on 3 year old gatemay laptop want to add kde
 

So where and how do I install kde with the mint 9 gnome i an running so i have both desktops.

You need to upgrade your distro, yours is old un-patched, and unsafe. Download The new version either 17 LTS or 17.1 then worry about KDE

thanks chris, so where can i do these upgrades while using the same,can it install while running.how and where??

You need to upgrade or install a new system. As you can see at the Linux Mint link below, Mint 9 has not been supported since April, 2013. Mint 13, 17 and 17.1 are currently supported and 13 and 17 are long term support.

http://www.linuxmint.com/oldreleases.php

The link below has some explanation of upgrading Mint, pros and cons. It would be easier to do a new install which they refer to as a "Fresh Upgrade".

http://community.linuxmint.com/tutorial/view/2

The link below is to the download site for the three supported version. You will be better off with 13 or 17 as there is longer term support. There is also a KDE version for each if that's what you want.

http://www.linuxmint.com/download_all.php