"Plato principal" if singular, "platos principales" if plural. And don't make the mistake of translating "entree" (Eng) as "entrada" (Sp), since in Spanish "entrada" actually means "appetizer".
A main dish is the main food that you are eating. On the other hand, a side dish is what comes between your main meal. For example chicken is your main meal, biscuits is your side dish.