ESXi is free, however when you have installed it you will see, 
Solution
1. To get your key go to the VMWare web site > Products > Download > Log in > It will give you a key with the download.
2. Connect to the ESXi host with your vSphere client.
3. Select the Host > Configuration tab >  Licence sd Features > Edit > Enter Key > Type in the key >  OK > OK.
 Published ByS.G.Godwin Dinesh.MCA
Sr.System Administrator