The problem with your object is its subtype: “Flow Device” is selected for it instead of “Flow Equipment”, so the MEP Add-On does not recognize it. If I change the subtype for “Flow Equipment”, save the object and place it again, the Pipe connection works fine.
There is no big difference between the […]MEP_4 and […]ACL_4 macros. The […]ACL_4 macro is more up-to-date, and it is shipped within ARCHICAD Library. The […]MEP_4 macro is used by the objects in the MEP Library, it is shipped within the MEP Library. As the ARCHICAD Library and MEP Library can be loaded separately, this macro appears in both libraries.
Library Developer, Library Team