‘package ‘XML’ is not installed for 'arch=x86_64'’

I bet Simon is right. Two main reasons:

1. I was following a blog post on how to set up FastRWeb on Mac OS X. This post (http://www.r-bloggers.com/setting-up-fastrweb-on-mac-os-x/) speaks about installing from sources and, even though "I think" I move to binaries prior to posting here on this issue, I can't be sure about that.
2. My understanding about multiple sub-architectures is more than poor. So, I could be sure I used the correct terms and concepts when analyzing the issue I was facing here.

I always try to stick to "official" sources of information when studying how to use a given R piece of software, but Google and other search engines put tones of other documents a couple of clicks away and is by no means easy to decide which one deserves to be followed. I think many of them can be useful when read with enough background as to be able to separate noise and information.

Please, allow me to hijack this thread to put in another, for me, related question I'm trying to work out on the list. To the best of my understanding and by following the documentation when I install R binary for Mac OS X both i386 and x86_64 are available. In fact I can easily choose among them by launching R Console by click on R or R64 "icons". But I'm not able to find the switch I must use to get the same thing when running R from the command line: I always get x86_64. I'm not able to solve this question by following R for Mac OS X FAQ or other sources of information available from CRAN.