Hi have just downloaded this game, and as soon as I open it, my mouse disappears, I cannot even enter a nickname. Any advice? At the moment I am pressing the Windows button on my keyboard to make the mouse reappear

It's normal for the mouse to not be visible at that point - on the next screen, after the doctor / training/ blackboard animation completes, the mouse will appear and you can select menu options.

Not sure why you can't type in a nickname though - you should be able to use the keyboard to type one in.