Hi Marco,
Assuming you're talking about the Virtual Center Server Appliance (and not the Storage Appliance), it's the same as the regular vCenter licensing.
In the vSphere client connected to the appliance, go to the "administration" menu up the top of the window, then select "vCenter Server Settings"
When you purchase a license you get a key which is placed in here. The download link is no different as vCenter is in a 60-day trial if no key is entered.
Cheers,
Book - Mastering VMware vSphere 5.5
Blog - LabGuides.com & NickMarshall.com.au
Podcast - vBrownBag.com
