I think Parry has it right. The button was created using a button from the control toolbox toolbar. It doesn't give you the option of assigning a macro from the right click menu. You have to run the macro from the buttons code(on_click) which is located on the sheet that the button is created.

A button created from the forms toolbar will give you the option on right mouse click of assigning a macro.

Two completly different things allthough they are both buttons. At least thats what I think.