I installed following P4V client version on my Mac and found that when I try to view difference of a file in change list it doesn’t open P4Merge,
Helix P4V version: Helix Visual Client/MACOSX1011X86_64/2018.1/1637591
I had searched over Internet to find resolution but all went in pain. Finally I tried to configure the P4Merge in P4V like below and to my surprise it just worked for me !
Note, you have default as P4Merge in P4V Preference option which was not working for me . All you need to configure is actual executable path with %1 and %1 as arguments.