FYI from JMP tech support:
If you are running JMP on a server, note that JMP 16 addins do not work for JMP 18 because the file paths have changed. JMP is no longer part of SAS so the file path changed from C:\ProgramData\SAS\JMP\addIns to C:\ProgramData\JMP\JMP\addIns
Changing your file path to C:\ProgramData\JMP\JMP\addIns will make addins visible for JMP version 18.