It doesn't work with Better Creature Descriptions , if you want it to work then you need to delete interaction_revised.txt from the Better Creature Descriptions folder

Eating a unicorn heart could provide some syndrome, bad thought or memory to certain dwarves, according to their personality. Maybe even trigger a fey mood. Something to reflect this as the really evil act it is. Otherwise, great mod, as are all your other mods.

I'd assume that you confine them to a space containing a limited food stockpile with the heart in it, either via burrow, military stationing or you could build a bedroom and bed and wait for them to sleep there then block the entrance door(s). Eventually when they would get hungry, they got the heart to eat.

how do I tell my dwarves to eat a heart? or will they do it one their on.

The regeneration is a one-time effect.

So, question. Is the regeneration provided by unicorn hearts a short-term/one off thing or is it a permanent effect and whoever eats it is permanently able to regenerate injuries?