The first thing to do is to make sure the software can find the mouse. Work out which serial port your mouse is connected to - usually this will be /dev/ttyS0 (COM1 under DOS) or /dev/ttyS1 (COM2). (ttyS0 is usually the 9 pin socket, ttyS1 the 25 pin socket, but of course there is no hard and fast rule about these things.) There are also an equivalent number of /dev/cua devices, which are almost the same as the ttyS ones, but their use is now discouraged. For convenience make a new link /dev/mouse pointing at this port. For instance, for ttyS0:
ln -s /dev/ttyS0 /dev/mouse