The product key you use will depend on how your licensing is setup.
For information on using KMS, please see this Microsoft article for information on what key to use and other information:
http://technet.microsoft.com/en-us/library/ff793421.aspx
If you are using MAK then you can use this information to load the key and activate the client:
To activate Windows 7 as a post installation task using your companies MAK, create a new batch script and copy the following information, substituting the "x"s for your product key.
cscript C:\Windows\System32\slmgr.vbs /IPK xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
cscript C:\Windows\System32\slmgr.vbs /ato
**Note, if you are using OEM or retail media then you will need to activate the machine with the product key that is on the computer or with your media. You can either change this task for each machine you deploy or just enter it manually on each machine.
**Note, you may need to install the Windows 7 Activation Patch as a post installation prior to this batch file. You can find the Microsoft patch here:
http://support.microsoft.com/kb/971033
Upload the file as an application item, and in the command line, put the name of the file with the switches /quiet /norestart