"Open keyboard settings..."
- Open System Settings > Keyboard (
or Command-Space "Keyboard")
- Select "Text Input" > "Input Sources"
> [ Edit...]

Add "ABC - Extended" keyboard
- There may be one or many keyboards 
on the left listed by country name.
- If "ABC - Extended" is already listed, 
then click [ Done ]
- Otherwise, click the [ + ] button on 
bottom left
- Search for "ABC - Extended"
- Select "ABC - Extended" on the right
- Select [ Add ] button
- Press [ Done ] and close the Keyboard 
settings window

Activate "ABC - Extended" keyboard
- Switch keyboards with the "global" button,
often bottom left of the physical keyboard
- Or select "ABC - Extended" from the 
keyboard (country code) icon in the 
top menu bar.

Entering Pali diacritics.
- Open any text editor
- Ensure "ABC - Extended" keyboard is active
- Press [ Option ] and [ x ] keys 
simultaneously then [ d ] alone to type ḍ
- Press [ Option ] and [ x ] simultaneously, 
then [ Shift ] and [ D ] 
simultaneously to type Ḍ

Some of the most common Pali diacritics and 
macrons are presented below. Similar patterns 
can be used for additional characters.

[ Option x ] [ d ] for ḍ
[ Option x ] [ t ] for ṭ
[ Option x ] [ n ] for ṇ
[ Option x ] [ m ] for ṃ
[ Option x ] [ l ] for ḷ
[ Option x ] [ Shift L ] for Ḷ

[ Option a ] [ a ] for ā
[ Option a ] [ e ] for ē
[ Option a ] [ i ] for ī
[ Option a ] [ o ] for ō
[ Option a ] [ u ] for ū
[ Option a ] [ Shift A ] for Ā

[ Option n ] [ n ] for ñ
[ Option n ] [ Shift N ] for Ñ

[ Option w ] [ m ] for ṁ
[ Option w ] [ n ] for ṅ
[ Option w ] [ Shift N ] for Ṅ

 

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Screenshots...
There are many ways to get to the Keyboard settings 
(with different names), but any Mac user should be 
able to find the Keyboard settings one way 
or the other...

After clicking Text Input > Input 
Sources > [ Edit... ]

After pressing [ + ] bottom left, the user will see:

Above I searched for "ABC" to limit the options. 
Then I select "ABC - Extended":

And press [ Add ]

Then press [ Done ].

Then at any time, to switch keyboards from
the top menu:

Note the "Open Keyboard Settings..." 
at the bottom. This is the simpler way 
to get to Keyboard Settings in the first step, 
however, the menu may not exist at all if 
the user only has one Keyboard layout configured.