My recipe calls for salted and I only have unsalted. I assume I can just add salt, but how much?
If for a baked item, I'd add salt quite sparingly - only if you really think you must. Otherwise, for anything else, just salt to taste. Unsalted butter tends to have a nicer flavour, so that's why it's recommended for cooking.
I use both salted and unsalted butter at work(baker/pastry) but the little bit of salt that is added usually doesn't affect the recipe. I would add 1/2 of the recipes recommended salt and taste it. if you feel that you need more, go ahead and add it.
The little bit of salt added to butter is there as a preservative and not a flavoring.