You can't sign up for those in advance -- on embarkation day, go to the location noted on your Navigator (probably Guest Services, though I've occasionally heard other locations mentioned).
Check your cruise meet here or on FB - many times someone will organize mixology classes or tastings ahead of time, so you can avoid the rush to sign up on embarkation day. Our group did several mixology classes and at least one martini tasting that we signed up for before sailing.
We've always signed up for tasting classes at Guest Services except on the Dream where the Navigator directed us to one of the bars to get tickets. If there's no specific location in the Navigator, go to Guest Services.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!