As far as I know, there are no universally recognized symbols for left-/middle-/right- mouse clicks, so the question of how to show such a symbol may be moot.
Usually, I just spell these things out, e.g.:
- Right-click the object you wish to edit.
- Type X then J to apply the effect.
If you are giving instructions and your individual steps are starting to look confusing because of all the click instructions, one thing you can try is expanding into more but simpler steps.
In the specific answer that you linked to, you have:
- Select the entry number (it's at the left of the command). You can paste it pressing Middle Mouse Button.
I would say that's a good case for just typing it out†. It will still be clear:
- Select the entry number (it's at the left of the command). You can paste it by clicking the middle mouse button.
And possibly dividing into smaller steps:
- Select the entry number (it's at the left of the command).
- Middle-click to paste it.
I'd stay far away from OOO or other attempts at a graphical representation in text, by the way. It is definitely unclear for a number of reasons that you have probably already considered.
As an aside, you may wish to consider ordered lists rather than unordered lists if your instructions consist of steps that should be performed in a sequence.
† Also, I can't find the "Middle Mouse Button" key on my keyboard! :)