libertine-legacy is listed and linked on the MiKTeX homepage, also libertineotf. Either your repository list is not current, or there's a bug in MiKTeX. Still you could install it manually, which can be a bit challenging with font packages, or consider using TeX Live.
Hi Stefan,
the problem is certainly caused by repos (italians, in primis) and maybe by my initial choice to use the Basic MikTex installation. What is the main procedure in such a condition? By trying to compile a code with a lack of packages (and being the internet connection active), MikTex provides automatically to search them in a close repository and loads the packages needed to the code.
As said previously, for libertine this procedure stops with some errors. What I've to try is to erase completely the actual MikTex 2.9 and install the full version. In case of further errors, of course, I'll switch to TexLive.
Thank you very much, Stefan, and regards.