The Liberation fonts have different names, and don't look quite the same as the Microsoft fonts (see http://en.wikipedia.org/wiki/Liberation_fonts
for comparison). They do have the same metrics though, so the flow of text on a page will be the same with either font.
The Microsoft names are aliased to their Liberation equivalents, but the aliases aren't listed in LibreOffice. If you explicitly type "Arial" into the font name box and hit enter, that should work (although the font used to display "Arial" will actually be Liberation Sans).