I do not understand what I do here:
- set XOOPS_TRUST_PATH into mainfile.php
If you've installed properly 2.3.1, then the XOOPS_TRUST_PATH is already set for you and it's the /xoops_lib directory (unless you've manually set it somewhere else).
For the other points, do the same as in case of pico:
- copy xoops_trust_path/libs/ in the archive into yourxoops/xoops_lib
- copy html/modules/altsys in the archive into yourxoops/modules/
- install it
For more info about the XOOPS_TRUST_PATH concept, please read this
Also, please remember that in 2.3.x, the Installation sets automatically the XOOPS_TRUST_PATH to yourxoops/xoops_lib directory. So if any module require you to copy any files to your XOOPS_TRUST_PATH, copy them to:
yourxoops/xoops_lib