The LaTeX3 Project have recently updated the organisation of the various LaTeX3-based packages on CTAN. This means that the older expl3 and xpackages need to be replaced by l3kernel and l3packages. Unfortunately, this seems to confuse MiKTeX, which does not pick up the need to install the new material. So MiKTeX users will need to do this by hand in the MiKTeX Package Manager. This should be a passing problem, but does seem to be causing some confusion for MiKTeX users.