mgbirder,
The photo shows the EXT. port of the 540 (sorry for the lousy quality). At the tip of the toothpick, in the groove of the port, is a spring-loaded switch. When an external power cord is plugged in, it pushes the switch in, telling the flash that there is an external power source.
The switch is stuck in the "in" position. You can use a needle, trying to push/pull the switch down and the internal spring will push the switch out.
If that does not free the switch, there is a simple workaround: change the charging mode to PP2. See the manual page below: