How do I get expl3 from CTAN?

In common with a number of other Linux distributions, Gentoo probably uses a rather old version of TeX Live (I would assume TL2009, but cannot be sure). You will therefore need to obtain the necessary support files from CTAN and install them locally.

The LaTeX3 support material is currently split into three parts

• l3kernel
• l3packages
• l3experimental

You will need l3kernel and l3packages to use siunitx. The links above are to the ready-to-install TDS-style .zip files. Download these, and unzip them inside your local TeX tree (usually ~/texmf), to create a structure. You should not need to run texhash for a local installation, unless Gentoo includes a really old TeX Live or if your ~ folder is not on the 'local' PC.

The 'experimental' nature of the code has been clarified a lot in the past couple of years. The l3kernel and l3packages code is stable for general use, while l3experimental may be more variable. However, none of them will have any impact on other LaTeX material (unless it uses LaTeX3).

If you are using MiKTeX 2.9, try updating it. On Windows:

Start > Miktex 2.9 > Maintenance (Admin ) > Update (Admin)

That solved it for me. I found this suggestions after crawling the web for hours. So, I hope it helps.

On Windows and for the case of MikTeX, it is possible that there are two installation: One under the user space and one for all users.

One solution could be that the user deletes all contents at

<<DRIVELETTER>>:\Users\<<USERNAME>>\AppData\Roaming\MiKTeX

and then tries to compile their LaTeX file that will ask to re-installing all required packages.