For one of your conditions, click "New action." Choose the object you have the Physics behavior applied to. Go to the Physics tab. At the bottom, there should be a Velocity section with an option that says "Set velocity." It will ask you to input the X and Y components of velocity for the object.
You can also read the object's velocity instead of setting it. If you want something to happen when the velocity in the x direction reaches a certain level, for example, you could do the following:
Insert event --> System --> Compare --> select the "Value 1" field --> select the object from the list of objects at the bottom of the window --> go to the Physics tab --> choose "Get velocity x component"
You can then choose the value to compare the x velocity to. When you're done, you'll have a new condition on the event sheet. You can then add actions to that condition which will execute when the condition is met.