Create a macro in the Global Macro Group (or some other group that is always active).
Add the USB Device trigger for the desired device being attached or detached.
Also add the Engine Launch trigger
Use the If Then Else action and the USB Device condition, check whether the device is attached or not, and then use the Set Macro Group Enable action to enable or disable the desired macro group dependent on the existence of the USB Device.
You can (and probably should) use the Activate/Deactivate Macro Group action instead of the Enable/Disable action. Enable/Disable is designed to be relatively permanent, and appears in the Keyboard Maestro editor whereas Activate/Deactivate is designed to be situation dependent.
A deactivated macro group will be functionally equivalent to a disabled macro group, and an activated macro group (assuming the macro group is enabled) will mean the triggers on the macros are available to be triggered.