In addition to the general problems listed by Burkhard Hoefling associated
with implementing external GAP packages on the MAC, there is unfortunately
another one involved in the "cohomolo" package, which is that the
principal external program is not a C program, but a rather lengthy and
quite complicated UNIX Bourne shell script, which calls the C-programs.
To get it it running under anything under then UNIX would therefore
require re-writing this script the appropriate Job Control Language.
Initially, I tried using GAP as the JCL, with lots of GAP Exec statements,
but this proved to be prohibitively slow and inefficient, since each call to
Exec starts up a separate UNIX shell.
I wrote this package as a standalone more than 10 years ago, and I don't
have any immediate plans to do any more detailed work on it, so I am
afraid that it would not be practical to implement it at all on the MAC.
Perhaps it would run as it stands under MacMint, however?