I suspect that OP is using the term "cpu" to mean the entire computer case with ports on the back panel. I'm sure he is NOT trying to plug a mouse into an internal CPU chip socket.
OP, if the mouse connector simply does not fit smoothly into the socket you are trying to use, do not force it - you could break it badly. Examine the connector on the end of the mouse cord AND the socket. Look at all the pin locations. Plus look at special grooves or bumps in the shell of the connector. See if all those pins and shapes match exactly.
There have been THREE common types of connectors and sockets for mice and keyboards. An older type common up to the early 1990's had a connector with a diameter of about 10 mm and a five pins in a half-circle. It was replaced by the PS/2 connector with a smaller diameter of about 5 to 6 mm, still with 5 pins but in an arc larger than a half circle. It also has a plastic tongue sticking out inside the arc of the metal pins, and hence a mating slot in the socket on the computer. And finally, the most common ones now are the plain USB2 connectors and sockets, which are rectangular about 12mm wide and about 5 mm thick with four metal contact strips mounted on a flat bar across one edge. You could not plug one type into another. So, which type of connector is on your mouse cord? Does it match the socket you are trying to use?
Another question. When you say it is "not plugging in", do you mean that you cannot physically get it to fit into the socket? Or, do you mean that you CAN plug it in, but it still does not work?