I can't find the serial number?

it's not clear with what suite/program you need help.

A. if older software that has a perpetual license,

1. check your account (if you purchased from or registered with adobe), https://www.adobe.com/account.html

2. use the message 1 link

3. if currently installed on an older computer, your serial number can be recovered from that computer

B. if you have a creative cloud subscription,

creative cloud does not use a serial number and creative cloud subscribers to not have any serial numbers associated with their programs.  creative cloud subscribers use their adobe id to activate their programs.

download and install the creative cloud desktop app (after signing in with your adobe id - the same one you used to pay for your creative cloud subscription),  https://creative.adobe.com/products/creative-cloud

open the desktop app (a  shortcut will be on your desktop) > click apps and then select the programs you want to install.

the desktop app gives you access to cs6, cc, cc 2014 and cc 2015 apps.  and probably more in the future.

if you already have the cc desktop app and you need to refresh your adobe id, sign out and then back in to your cc desktop application using the same adobe id used to pay for your subscription:  http://helpx.adobe.com/creative-cloud/kb/sign-in-out-creative-cloud-desktop-app.html