Hi niko, like NightCabbage says you can find it on your old computer where you've got it installed under either that folder or if you've got Vista or Windows 7 on the old computer it's under this location instead:
C:\Users\[your user name]\AppData\Roaming\Moi

There will be a file there moi_v2_license.key that contains your license key, you can open up that file with notepad to view it.

If you can't find it that way, you can send me an e-mail and I will look it up for you in the license database. For that just e-mail me at moi@moi3d.com and let me know either the Plimus order number or the name and e-mail address that you ordered with and I can look it up for you and e-mail it back to you.

- Michael