Unlike 3D Studio MAX, which provides a grace period of 30 days, you must authorize character studio in order to use it.
The first time you activate any of character studio's tools, such as Biped, Crowd or Physique a dialog box will appear asking you to enter your authorization code.
Also displayed in the dialog is your application ID number. You will use this number to obtain your authorization code.
You can obtain your authorization code by going to the web site: https://register.autodesk.com and selecting 3D Studio MAX from the product list and your country from the country list. Be sure to add the "s" in https. Follow the instructions on the pages that follow.
In most worldwide locations, you can also call or email your application ID number to obtain the authorization code. Follow the instructions found in your product box to obtain your local phone number to call.
Note: The authorization code is specific to your 3DS MAX 3.1 hardware lock. If you change to a different hardware lock, you will need a different authorization code. It's a good idea, when you receive your authorization code, to write it on a piece of paper and tape it to your hardware lock. If for any reason you need to reinstall the software, you'll have the code readily available.