Let’s make your question a little more generic, shall we, so that it also speaks to people who aren’t making a Betty Crocker cake mix at this very moment: Can you substitute unsalted butter with reckless abandon for salted butter?
If you don’t add a little salt to compensate, though, your cake may taste a little bland to you. Your food should never taste salty – just enough salt should be added to bring out the flavor of other ingredients. Salt was originally added to butter as a preservative, but in this country we’ve just gotten used to the taste of salted butter, and it has become the standard.
Your Betty Crocker SuperMoist Butter Recipe Yellow Cake Mix already has some salt in the mix, but it’s the 10th ingredient, so there is probably not a ton. The instructions call for you to add 1-1/4 cups of water and 3 eggs, as well as 1 stick of butter, and although it doesn’t specify salted or unsalted butter, the manufacturer assumes you have salted butter on hand.
If you want to compensate for the use of unsalted butter, add as much as 1/4 teaspoon of salt for every half-cup (stick) of butter called for in a recipe. If you use kosher salt, which is less salty for a given measure, you may want to add a bit more.
Many serious cooks bake with unsalted butter as a general practice, because it gives them control over how much salt they add to their recipe. That could also be important with processed foods, which often include a lot of salt, but is probably not important, because they seldom call for the addition of butter.