How can molecules be polarised covaltently bonded by with no dipole?

Answer 1

These molecules can be temporarily polarised, but not permanently. Look up van der Waal's forces.

Answer 2

There will be no dipole if the central atom has a partially positive of negative charge while the outer atoms have the partial opposite charge.

ex: CCl 4 the central C is partially positive while each Cl around it is partially negative. They have polar bonds but the geometry of the molecule does not let it be a polar molecule.