The NIC type/model to use depends on the operating system and the drivers available. When you create a new virtual machine, it is configured with the recommended NIC type for the selected operating system. However, the vmxnet adapter may provide better performance, and an appropriate driver is included in the VMware Tools.
As for adding the license. The way you describe is only available when you connect the vCenter Server. When you connect directly to an ESXi host, follow the steps for "Licensing a standalone ESXi host" in http://kb.vmware.com/kb/2014295
André