the error message appears to be self explanatory: My_Custom_Menu will be supplied with one argument that is a disembodied property list ("dpl" for short) but it was defined to accept no arguments:
*Error* My_Custom_Menu: too many arguments (0 expected, 1 given) - ...
The trigger will automatically supply the dpl to the function, this is why the function needs to accept at least one argument. See my example where I have "args" for this argument, and then this is used to obtain the window id (this is the correct way to do it anyway, since hiGetCurrentWindow() can actually pick up the "What's New" window, for example).
Also, which version are you using? I'd guess at 9502/4.3.4 given the "dag" prefixed functions, but it is possible that these are just customizations...
Try again and let us know how things turn out.