If I have meniere's disease, what should I do during an attack?

Medication. My preferred medication for someone having a meniere's attack is valium. It controls the vertigo quite well, sometimes i will also use a diuretic such as hydrochlorthiazide. If you have meniere's you should also have an ENT doctor who can follow and treat you.
Stop, rest. Stop, find a safe place to spend while the attack runs its course. If you have and are able to swallow either a xanax (alprazolam) or valium tablet, take it.