Help installing photoshop cs5 extended

I have photoshop cs5 already installed.  I don't know how to install the extended part?  I put in the serial code but it did not come up.  Do I have to deacivate photoshop cs5 to make it work?  When I install photoshop cs5 do I put in the extended serial number or the other serial number?  Both serial numbers work that is not the issue.  I hope someone can help me out here.