The rules state that empowering a unit will give it an extra attack strength, counter strength, and toughness value.

But it doesn't say anything about removing an activation token (if the unit already has one). (at least I didn't spot this in the rules)

However, the preview for the Empire implies that if Volkmar empowers Karl Franz then Franz would be able to use his focus ability again (which I interpret to mean that the empowerment would remove an activation token from Karl Franz). Am I interpreting this correctly? If so, am I missing anything in the rules where it mentions this aspect of empowerment?

Empowering does not remove the activation marker. I wish I could find the section of the preview that caused this confusion so we could try to interpret what was meant.

My bad, upon reading the section closer I understand it now. In the preview 'Assemble the Hoards' it has:

Alternatively, you might pursue a different strategy and pair Volkmar the Grim with Karl Franz . While Karl Franz wouldn’t gain Frenzy and Resistant ( , ) like Luthor Huss, he could remove activation tokens from Volkmar and another one of your units. In this way, your Hellblaster Volley Gun could fire twice in a round, and Volkmar could empower two different units.

I misread this the first time, it is just saying that the focus ability of Karl Franz can act upon Volkmar and another unit to allow them to perform an additional action.

At the end of Turn section, playes remove all tokens except Wound markers, so empowerment only lasts for the turn.