What's the correct install direction for slotted rotors?

Mounting the rotors in the correct direction has nothing to do with the slots on the sides. You mount the rotors so that the cooling vanes inside sweep out when spun in the forward sense. The idea is to fling the air in them toward the outside using centrifugal force, and suck cooler air in with the vacuum created by the exiting air.

Check your vanes, if they are straight, it doesn't matter how they go on, but if they curve, make sure that when the wheel rotates in the forward sense the outer edge of the disk isn't scooping up air.

EDIT:

For clarity, here is a render of a rotor I've done, with one transparent side, showing the vanes. This rotor should rotate in the clockwise sense as seen.

Really? I like the look of the opposite way. Looks like claws *does claws pose with hands*

or something like that. Your way looks like it would keep stuff out of the slots better though, fling it out the end of the slot, if there is any difference.

yours are on right same type principal with the cooling vanes, they need to pull the dust away from the center