Personally scanning for PUPs can bring more pain than gain (why it is off by default) as the user has to have a reasonable knowledge of what a PUP is, what is installed on their system and what it does, so when asked the question (essentially what to do) they can make an informed decision on what action to take.
Scanning for PUPs happens in two different areas:
On-demand scans - in which case you need to change the Settings, Sensitivity, 'Scan for potentially unwanted programs (PUPs), for the scan you want to run (Quick, Full, etc.).
Real-Time Shields - Expert Settings, Sensitivity, 'Scan for potentially unwanted programs (PUPs), for the real-time shield you want, File System Shield, etc.