Normally you install vCenter and then the license option is available where you are expecting but here is VMware's info on licensing it without vCenter installed.
License an ESXi Host Without vCenter Server
If you are directly connected to the host through the vSphere Client, you can assign the host a license key.
Note
Verify that the edition of the license key you assign to the host matches the currently configured features and resources. Otherwise, you cannot assign the license key.
Procedure
1 From the vSphere Client, click the Configuration tab.
2 Under Software, click Licensed Features.
3 Click Edit.
4 Assign a license key.
Select Assign an existing license key to this host and select a license key from the Product list.
Select Assign a new license key to this host, click Enter Key, and enter a license key and an optional label for the license key.
5 Click OK.