I have my registration key, have already registered, why can I not access the full operational Synplant? It just shows me the tutorial...

The tutorial starts by default on both trial and full version. You can close it with the [X] down in the corner. Once you have gone through the tutorial, it should remember that and not display it again.

If it keeps restarting the tutorial every time, then Synplant cannot save its preferences for some reason.