Given that the laws of physics are symmetrical with respect to time there should be no reason for the past to be any different than the present, but it obviously is. Thermodynamics says that as things change they tend to change into states with more disorder. That can be stated without appealing to any particular time direction. Time appears to have a consistent direction because we originated in a region of the universe with above-average degree of order and are now progressing into less-ordered states (on a medium scale -- on the small scale of humans order can increase noticeably). This is all just due to random fluctuations in entropy throughout the universe. Eventually we'll just be an undifferentiated soup with no clear direction being "forward in time" or "backward in time." Listen to Feynman's very interesting lecture on this topic.
As far as the meaning of "now," that's just a word we made up to describe things that are immediately present in our consciousness, i.e. not memories but actual perceptions of reality. This is not actually a point -- "now" is actually a range of times and events which are all immediately pertinent -- and the radius of "now" depends on who you are, what you are doing, the details of your specific brain, etc.
