xAct installation on Macintosh:
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1/ Control-click the Mathematica application and choose
"Show package content" to open it as a folder.

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2/ Open the subfolder AddOns/Applications and copy there
the uncompressed xAct folder.

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3/ From a Terminal, go to the subfolder
AddOns/Applications/xAct/xPerm/mathlink
within the Mathematica bundle. Type

pwd

and copy the long path

/[...path...]/AddOns/Applications/xAct/xPerm/mathlink

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4/ Edit the file xperm.tm (located within this directory
AddOns/Applications/xAct/xPerm/mathlink) to change the
path of the xperm.c source. The corresponding line is at
the top of the file, just below the /* PACKAGE */ comment:

#include </[...path...]/AddOns/Applications/xAct/xPerm/mathlink/xperm.c>

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5/ Still from the Terminal and within the same subdirectory
AddOns/Applications/xAct/xPerm/mathlink
type the command (on a single line)

../../../../MathLink/DeveloperKit/Darwin/CompilerAdditions/mprep
xperm.tm -o xpermtm.c

[This command should work if the executable "mprep" is located
in AddOns/MathLink/DeveloperKit/Darwin/CompilerAdditions/
You can check this by typing
ls ../../../../MathLink/DeveloperKit/Darwin/CompilerAdditions/
If the names "mcc" and "mprep" are not printed, you should look
for these files in your Mathematica installation and change the
path accordingly in the above command line.]

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6/ Type now (still on a single line)

../../../../MathLink/DeveloperKit/Darwin/CompilerAdditions/mcc
xpermtm.c -o xperm.mac

This creates the executable "xperm.mac"

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7/ If you got the error message that "libML.a is out of date"
at step 7, just type

ranlib ../../../../MathLink/DeveloperKit/Darwin/CompilerAdditions/libML.a

and re-execute the command line of step 6.

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8/ Double-click the file

AddOns/Applications/xAct/Documentation/English/xTensorDoc.nb

to open it in Mathematica, launch xTensor from the section
"Load the package", and test section "0.1. Example session"
notably the line

Simplification[%]

to check that the C executable "xperm.mac" is indeed used.