Early versions of the driver improperly write certain information to system files when the background printing drivers (Print Monitor, Backgrounder) are present. The result is that the Print Monitor, Backgrounder, and the System file are corrupted. The damage is permanent, and cannot be corrected until you first remove, then re-install your system software.
Until you receive the new version, delete the files Backgrounder, Print Monitor, and System from your system folder, then use Apple's Installer program to restore the System. Before using the MacPlot driver again, remove the fresh Backgrounder and Print Monitor files that were copied to your system folder during the installation process.
Note: All Chooser devices modify the System file in some way. MacPlot 1.1 writes information properly for System software versions 5.0 and earlier, but does not write the information properly when System 6.0 or later is used.