in the descri[ption of the possession power, it says that force is added to physical attributes. What about movement? does being possessed make you move any faster?

No.

Disregard, Slithery beat me to it. Note that Possession-enhanced Strength may help boost sprinting speeds though.

If someone is possessed by a Spirit of Air, can the dual entity fly?

SM says the combined being 'boasts all of the [spirit's] powers and skills'. While there isn't a distinct Power for a materialized air spirit's ability to fly, it's certainly something slightly different from the standard 3-Dimensional movement allowed by any materialized spirit.

And let's not forget that some spirits have the movement power too, which would solve that 'does a spirit make me move faster' question in a different way.

That's an amusing thought though, becoming a bonified superman via a spirit of air....XD

Power Pact: Movement