I found my DC-In cable VERY difficult to detach. I depressed the locking mechanism, but still needed to use my iFixit Jimmy to carefully, gently, slowly work the cable connector (male) loose and away from the power supply (female).

This was not easy to disconnect with one hand holding the power supply (being careful while holding just to be cautious against any possible electrical discharge). Note that the connector is not on power supply side but on AC-In cable side. Couldn't quite see well enough from photo shown, but hold on the AC-In side of cable connector (depressing the locking mechanism of the male side) and pull away from the Power Supply (female side) cable.