Based on recollection, and considering it has been a decade since I used a pre-facelift W212, Parking Assist would signify a prospective parking space on the left side of the vehicle as you gradually pass a location by putting a triangle next to the left of the 'P' inside a box symbol.
Cease movement and engage the vehicle in reverse. The system will enquire if you want to activate Parking Assist: hit OK to proceed. The display will thereafter show the direction to steer when you remove the brake. It will also indicate when to cease and engage Drive before subsequently directing the appropriate steering direction. The procedure continues until you have successfully parallel parked.
The system will identify a parking space to the right of the vehicle, but only when the right turn signal is activated while gradually approaching a possible opening.
The subsequent models, including the facelifted W212, have Active Parking Assist, which autonomously steers during parking manoeuvres and can detect and automatically reverse into a perpendicular parking space.