External Tools >> Revision Control Diff/Merge Greyed Out

Hello I am attempting to change my Revision Control Diff/Merge on my Mac from Apple File Merge to Beyond Compare, but it is greyed out and not letting me change it. Does anybody know why this is? Here is a picture of it.

Diff tools are detected in their default installation locations (and using registry keys for TortoiseMerge, WinMerge, PlasticSCM Merge, and Beyond Compare 4 on windows).

Have you installed it somewhere else?

I’d like to use Visual Studio vsDiffMerge tool for diffs but I cannot set it in Preferences.
Is there any way to a set a custom diff tool in Unity?

thx a lot in advance.