Before installing the PortDoubler software you need to temporarilly deactivate any software that could be using your serial ports. Start by deactivating or turning off the software of the devices you will be connecting to the PortDoubler as well as your other serial port. This is to prevent two pieces of software from trying to use the same port at the same time which may cause the Mac to freeze. The most important software to deactivate (if it applies) is
the fax software, PaperPort software, ARA server, and File Sharing.
Consult your documentation to locate these on/off switches. They are usually located within Control Panels or their own setup software. Here are some examples: