how to migrate aperture 3 (purchased and downloaded online) to new macbook.

OK. On your new computer, sign into the Mac App store using the same AppleID as you did to purchase Aperture on the old computer. Go to the "purchases" tab and select the "install" button next to Aperture. This will install a new copy on the new machine. Now, go to your old computer, and in Aperture, select "File->Vault->new vault" and create a new Vault to backup your images. Store this Vault on an external HD, and once the backup is completed, exit Aperture and eject that disk form your old computer. Attach it to the new computer and in Aperture, select "File->Vault->Restore Library" at which point you can select the Vault you just made. This will restore the library you just backed up onto this new machine.

The point is: I want to deinstall aperture from old mac and install it in my new mac, but as i downloaded it online, i never needed to set any license number (not that i remember). What can I do??

just an add-on:

Why do you want to uninstall it from your old mac? If you bought Aperture from the AppStore for private use, the licence entitles you to install the application on all Apple-branded computers that you own or control. So you can install and use it on more than one of your macs.