Hi,
The collision event works on the object that the fsm is on
The Set velocity is standard on 'use owner' but you can change this by selecting 'specify game object' then you can drag in an object or use a variable.
So if you have it on the blocking object you need to 'specify game object' and drag in the movable character
If its on the movable character you can leave it on 'use owner'
also be sure that your set velocity on the start is not set to every frame.