Locate your Amnesia game folder under your "My Documents" menu, then open the file
"user_settings.cfg" with your preferred text editor. Search for the line that reads
"AllowQuickSave="false" and change it into "AllowQuickSave="True", then save the 
user_settings.cfg, remembering to retain the file extension (*.cfg).
The default key for quick saving is [F4] on your keyboard.