There have been a few questions regarding installing this package in the past, but they now seem to be out of date. I'm using MiKTeX 2.9 and TexMaker. When I try \usepackage{mathtools} in TexMaker it will try to install the package for me but will fail. I took the advice given in this thread:
How to install mathtools package?
As suggested I tried installing the mh package using the MiKTeX Package Manager, unfortunately I get the "unknown archive size" error, which I have read means that the package no longer exists. Another thread (lost the link) said you can use the MiKTeX Package Manager to directly install mathtools, however I do not see mathtools listed anywhere in the MiKTeX Package Manager…
So I ask, is the package under a different name? Are there any other ways to install this package?
Best Answer
It seems like there is a bug in the MikTeX 5105 basic installer. Using it ("basic-miktex-2.9.5105"), i encountered the issue stated here; the "mathtools" and "breqn" packages do not appear and the "mh" package installation fails with the error mentioned earlier.
To circumvent this problem until a fix is released, download the net installer instead ("setup-2.9.4503"); it's version 4503 which seems to work just fine.