You are running esxi 4.1 and want to know how and where you install your license key.When you initially download and install esxi you get a temporary license of 60 days, when it expired you will need to install a valid license. This is simple but if you do not know your way around the esxi console then you may struggle to find it.
Once you have downloaded your copy of esxi you then need to create and account here
once done you will then get access to your license key.
Then , login to the esxi console with the vsphere client and select your host.
Once selected, on the right hand side you will then see the esxi options tabs. Select Configuration.
Then down the left hand side select licensed Features.
Vmware esxi is now licensed.