we created a splash screen before the main game stage but I understand that the mouse/keyboard is not active until I proceed to the main stage . What should I do in order to activate the mouse/keyboard before the main stage.

Further Info : My splash screen is on function main() and on "enter " key press I wish to proceed to the main stage(which is loaded inside function main1()).

thanks in advance

What do you mean by "main game stage" ?
Use the manual and read the pages about mouse_mode. Your second issue could be solved by
while(!key_enter) { wait(1); }
level_load("look in the manual.wmb");

thanks a ton.