LaTeX Error: File `subfiles.sty' not found

First of all, it is important to know how did you install TexLive:

• As a package from your linux distribution.
  • What distribution?
  • What version of TexLive?
• Directly from TexLive.

My installation of TexLive is the second one, TexLive 2012, and the file subfiles.sty is located at:

$TEXLIVE_HOME/texmf-dist/tex/latex/subfiles/subfiles.sty

So, if you installed from your distribution, you have two options:

• If using Debian (or Debian based distros), install texlive-latex-extra as pointed out by @egreg.
• If using other distros, try to search a good match with the respectiv package manager.
• If you prefer to have a more up-to-date LaTeX distribution, think about installing TexLive2012 from the link above.

UPDATE 1:

As you are using Linux Mint, this problem is very strange, because if you execute in a terminal:

apt-cache show texlive-latex-extra

you can find subfile package in the list.

UPDATE 2:

According to packages.debian.org, the location of subfiles.sty should be this:

/usr/share/texlive/texmf-dist/tex/latex/subfiles/subfiles.cls

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Not your desired solution, but my humble advice:

I use Linux Mint debian edition at home (and Debian at work), but on both systems I have TexLive2012 installed by hand, in a custom directory, so I cannot tell you what happens with the distro packages.

Haven't you thought about installing TexLive directly with install-tl?:

• Download install-tl and unpack it.
• Follow the instructions at TeX Live - Quick install.

I hope this can help.