Sorry if you already know this.
They are meant to pull out from each side of the engine. Don't try to knock them through towards the middle of the engine.
They shouldn't be very tight. Can usually just push with your fingers, grabbing them underneath the engine.
If they're tight, you should be able to use a wooden drift and mallet to gently tap them out. Because the MC21 lower mounts are two-part, just locate the drift on the inner end of one mount under the engine.
If they still won't budge, and you're changing them anyway, you'll have to use a bit of squirty lube and a set of grips to twist the bleeders to free them.
Grip them on the tubes under the engine. Just be careful not to damage the cases!
Good luck!