If you are going to use time out you need to pick her up and put her in time out not just tell her to go there, no two year old is going to sit on a step by choice! Keep returning her if needs be, gently with no shouting but with a clear message that your the boss. If you do this consistenly and don't back down in a few weeks she will go when told because she knows if she doesn't you will get up and take her there. Also ensure time out is a place to calm down and/or think about what she's done rather than a punishment, then, when she's older, she may start to choose to go to time out when she needs a moment to get her head together.
Personally with time out at this age I find it better to remove them and cuddle them the moment they calm down than to stick to a rigid time. As she gets older you could introduce a specific time she has to sit there (as sometimes we need a moment to calm down too!)Talk to her afterwards to ensure she understand exactly why she was sent as sometimes they can be confused as to which bit got them in trouble.