For me, it seems to be quite clear what OP wants / needs. A mouse press on the PC arrives (in some way) in the Arduino and the Arduino sends a character.
That first part would be similar to the getLeds() method which (I think) interrogates the PC. I do not know enough about USB (read: nothing) to be able to say if the first part is possible without additional software on the PC.
Now the question is why the OP wants to use an Arduino to achieve this. It would be a million times easier to detect a mouse press in software on the PC and and let that software type the character (e.g. on the screen).