Let me give a fully general answer, in case others need this information too.
If you have MaTeX 1.6.1 or earlier (which does not use paclets), it is located in FileNameJoin[{$UserBaseDirectory, "Applications"}]. Navigate to that directory and delete the MaTeX directory within.
If you have MaTeX 1.6.2 or later, it uses the paclet system. It is not strictly necessary to uninstall earlier versions when upgrading. <<MaTeX` will always load the newest version that is installed.
To see which versions you have installed, use the command
versions = PacletFind["MaTeX"]
To uninstall a specific version, apply PacletUninstall to it. For example, to uninstall the second element returned in the versions list above, use
PacletUninstall[ versions[[2]] ]
To uninstall all versions of MaTeX at the same time, use
PacletUninstall["MaTeX"]
I do not usually delete the files in ~/Library/Mathematica/Paclets/ manually. I believe it is better to let the paclet manager do it.
Some of this information is available at the following locations:
