Please can someone help me?
Got a trial version 3.1, but installed Zbrush is requesting the serial number, so I could respond with the activation code.
Turns out I’m not finding the serial number, can someone tell me what I should do?
Just informed me automatic code 2807-74FE-41D2-47cc.
The trial version is not activated. When you launch it, choose the option to “Activate Later”. This option will remain available for as long as days still exist in the grace period. Once the grace period ends, it is not possible to run the trial anymore, or to restart it.
Thank you for your quick response.
But still with the problem, because the options that appear, “activate later” is not enabled, only show up the following options:
Activate on line using your serial number.
Send e-mailed activation request.(I just do it but no have response).
Activate by phone.
Enter e-mailed activation response code.
(0) Grace days remain to activate Zbrush.
Thanks.
This means that ZBrush has previously been installed on that computer and was last run more than 30 days ago. The trial begins from the time that ZBrush is first launched and goes for 30 calendar days. Reinstalling ZBrush will not reset the timer. It is not possible to run the trial again on this computer. There is no way to restart the grace period.
Dear Aurick.
Thank you again for all the help.
Sorry if my English is not very good because I am also making use of an electronic translator.
I decided to buy Zbrush 3.5r3, is already installed and activated, functioning well.
I no longer know how to thank you, but if you come to Brazil someday, I will gladly welcome you.
Hugs
Welcome to the ZBrush family. 
Since you’re from Brazil you should look up Kris Costa (antropus). That’s some real inspiration for you!