In ArchiCAD, go to Options > Work Environment > Keyboard Shortcuts
Arrange the list of commands in alphabetical order, scroll down to 'Relative Coordinates in Tracker' and assign an unused shortcut key to it. This will actually affect both the tracker and the coordinates palette.
Firstly, make sure you are not trying to change it in the middle of a command.
To check it works, turn on the coordinates palette (Window > Palettes > Coordinates) and see if the triangles to the left hand side of the numbers change when you press the keyboard shortcut.
If it doesn't, double check the keyboard settings again!
Still not working. In your coordinates window, does your "Absolute/Relative" coordinate button, to the left of the coordinate entry boxes, change (from on to off) when you press your shortcut?
Yes, the triangles are highlighted like a button when relative, and plain when absolute. Repeatedly pressing the keyboard shortcut should cause them to flash on and off.