Getting the chair to move is easy (set move route: away from player), however, before you use the move route, set two different variables to the event's coordinates. I don't have Ace with me at the moment, but I think it's on the last tab perhaps. Or perhaps it's an event command on its own (get event location, I think). Use these to get the coordinates of your event. Check what the coordinates are for the spots you want the player to get on the chair and grab an item (click on the event spot, and the bottom right corner should give you two numbers, the first number is X and the second is Y), then use these in a conditional branch. By that I mean, check if variable X is the same as X, and then another conditional branch that checks if variable Y is Y.
If both true, make the player get on the chair and grab whatever. If false, make the event move one tile away from the player.
I can't give you a sample event, but maybe someone else would be so nice enough to do it for me.