I'd suggest Uber, a taxi or a city shuttle service from the airport if you're not hiring a car.

Self-drive directions from the airport

Follow the N2 (direction Cape Town).

Keep following signs towards stadium, Waterfront and Sea Point.

Just after you pass the hospital, a big Virgin Active and the CTICC, get into the middle lane.

The road will become Buitengracht Street.

Stay on Buitengracht Street and go through the first 2 traffic lights.

At the third traffic light take a right onto Somerset Road

Then the second right turn onto Alfred Street.

The Rockwell is situated on the corner of Prestwich and Alfred Streets.