Amante is pretty much right on!..
Basically Mr Right is the guy she knows she could stay with for a long time and someone who offers stability and a happy simple life for her. Often women avoid or sabotage these types of relationships. Most believe they have found him during their 1st serious relationship, but 99% of time they get their hearts broken and avoid these types of relationships until after college or when they are established and matured enough to appreciate this kind of guy.
Mr Right Now may be a jerk or bad boy who is exciting and fun for her to be with. She knows these relationships usually don't last so most times, women between 18-26 aim for these types of men.
hope that helps