You need to go into the installers mode menu first. If the code has been set to 0000 nothing will work. Once in the installers mode menu you go to define comm,press OK then cycle through the options until you get to Mast. DL Code, press OK then you should be able to change it.
One last thing did you install the alarm if not then someone may have set a code to access the installers mode.
If you go to the Visonic website. youcan download the installers guide from there, and this will show you in better dtail how to change the download codes. You will need the master download code as the user one doesn't allow 2 way comms. The user download code only allows downloading to the panel.
Personally I would go the RFXCOM route.
Many people have tried interfacing directly to the Visonic panels but without great success. There was a thread or two a while back (its about 6 years old now) on the Homeseer forums about this.