Quit It is not officialy supported on System 7.5 and earlier, but you can get it to work (at least with 7.5) in the following way:
• Put the Quit It control panel in the control panel folder. Note that dropping it on the system folder will NOT automatically put it in the control panel folder on System 7.5, because it appears as an application.
• Next put the Quit It Init extension in the extensions folder.
After a reboot Quit It is active, and you can change settings using the Quit It application.
If you register Quit It via the application, you will still get the shareware notification every now and then. You can get rid of it by using ResEdit to execute the following steps:
• Open the registered Quit It application in ResEdit
• Select the STR resource icon, and copy it to the clipboard (Cmd-C)
• Open the Quit It Init extension in the extensions folder in ResEdit
• Paste the copied STR resources (Cmd-V)
• Save and close the Quit It Init extension
• Close the Quit It application (no need to save as you shouldn't have modified anything)
After a reboot you should no longer get shareware notifications.
All this is not very elegant, but to make it fully compatible with System 7.5 is not worth the effort.