You should have send your .SER file and have received a key installer (for PC) from Acustica Audio. Did you received first time? Did you backup it? If not request now! Key installer is just 2 files INIT.N2P and INIT.N2V that you or the installer should overwrite over the demo key files INIT.N2P and INIT.N2V.
Nebula use a challenger response anti-copy authorization system. After first run Nebula will generate a .SER file in the plug-in support installation folder called NebulaTempRepository folder (in PC this path can be changed in the installation setup however is fixed in MAC and the path is Mac HD/Library/Audio/Presets/AcusticaAudio/Nebula3/). This file should be sent to Acustica Audio to generate a custom customer key, you can send in an attachment to email@example.com or upload in user area, my authorizations. A custom customer key will send from Acustica Audio for replace the demo key, removing any restriction so you will able to load any emulation preset. Be aware that the customer key is the INIT emulation preset that Nebula use for initialized and that means INIT emulation preset, customer key and INIT.N2V and INIT.N2P are the same thing.
Enrique Silveti. Acustica Audio customer and technical support.