In general, you should search for the base package name in your package manager. For example, libXp.x86_64 you'd search for libXp by itself. You can do this in pamac's window or from the command line:
pacman -Ss libXp
and you should be able to find the package to install either way.
The packages that have an x86_64 packages are going to be 64 bit and are the default packages you find and install. If the extension is i686 then it's a 32 bit package. You can find that by searching for the package base name and 32:
pacman -Ss libstdc++ 32
Most of the packages should in the repositories. If you don't find one, check the AUR using pamac, or another AUR helper.
I did a quick check and I think I found everything except the
gcc-g++ package, which I think is part of the gcc package and the two
compat packages, which I think are also parts of their respective packages. If I'm wrong about that, hopefully someone more knowledgeable will answer.
Check the rest of the tutorial so that you can adapt it to Manjaro. In particular, I don't think the symlinks are needed as we have libraries in /usr/lib already. If the install requires dash, it's in the repo.