Use this:
$('#navigation ul li').css('display', 'inline-block');
Also, as others have stated, if you want to make multiple css changes at once, that’s when you would add the curly braces (for object notation), and it would look something like this (if you wanted to change, say, ‘background-color’ and ‘position’ in addition to ‘display’):
$('#navigation ul li').css({'display': 'inline-block', 'background-color': '#fff', 'position': 'relative'}); //The specific CSS changes after the first one, are, of course, just examples.