If this is the Windows 8 Enterprise trial at the link below, then you do not need a product key number to install and use it for the 90 day evaluation period. After the 90 days, you would need to purchase a copy or product key of Windows 8.
You are not trying to install Windows 8 Enterprise if it's asking you to enter a product key during installation.
Activation and Usage Guidelines
Back up your files and settings before installing this evaluation and again prior to the 90-day expiration.
Upon installation, Windows will prompt you to activate. A product key is not required for this software.
You must complete activation before August 15, 2013, to use this evaluation.
Once you activate, you have up to 90 days to use the software. You can track the amount of time you have left by referring to the "watermark" in the lower right corner of the Windows desktop.
If you fail to activate this evaluation after installation, or if your evaluation period expires, the desktop background will turn black, you will see a persistent desktop notification indicating that the system is not genuine, and the PC will shut down every hour losing unsaved work.
Following the evaluation period, you will need to replace the operating system on your test computer and reinstall all your programs and data. It is not possible to upgrade the evaluation to a licensed working version of Windows 8. A clean installation is required.
In addition to Brink's info here's some extra stuff regarding activation of W8 ENTERPRISE.
This information is only relevant for W8 ENTERPRISE version.
Windows 8 Enterprise -- the FULL version has to be activated AFTER installation -- you enter a command in ADMIN mode SLUI 3
Windows 8 TRIAL shouldn't need a serial number (can't remember now if that's true) but if it does then enter SLUI 3 as above and then enter the serial number you got from the download. (You don't need to enter ANY serial number when you install Windows 8 enterprise --full version or trial).