If you have upgraded from an earlier version of the plugin, any existing Mobipocket PIDs will have been automatically imported, so you might not need to do any more configuration.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a green plus sign (+). Clicking this button will open a new dialog for entering a new Mobipocket PID.
Click the OK button to save the PID. Or Cancel if you didn’t want to enter a PID.
On the right-hand side of the plugin’s customization dialog, you will see a button with an icon that looks like a red "X". Clicking this button will delete the highlighted Mobipocket PID from the list. You will be prompted once to be sure that’s what you truly mean to do. Once gone, it’s permanently gone.
Once done creating/deleting PIDs, click Close to exit the customization dialogue. Your changes wil only be saved permanently when you click OK in the main configuration dialog.