You click on ARCamera then camera.  In the inspector on the right you'll see a window that says App Key.  Copy & paste your key into  the window. 

Hello,

For the iOS VuforiaSamples, grep for "setInitParameters". I found it in .../VuforiaSamples/Classes/SampleApplication/SampleApplicationSession.mm.

The key you create on the portal is the string in the second argument:

Vuforia::setInitParameters  (int flags, const char * licenseKey)

Thanks.