Using carnation famous fudge recipe, when I add the marshmallows, chocolate chips and nuts it gets thick and the marshmallows are not melting completely. Am I cooking it too long or not long enough, or do I need the temperature higher or lower? I have it medium heat. Stirring constantly to full rolling boil, and then cooking for 4-5 minutes stirring constantly.
Most likely you need to turn up the temperature a little before adding the new ingredients. All those ingredients are about room temperature, and there are a fairly large quantity of them compared to the mixture in the pan. This means that they cool down the mixture considerably when added. The cooling means that the temperature is not sufficient to melt marshmallows any more, and the fudge begins to set (thicken). Increasing the temperature slightly will help the marshmallows still melt.
The fudge is cooling too quickly but heating the syrup further is risky.
During the full boil you are boiling out water to reduce the sugar syrup to a specific concentration that cools to the fudgy texture*. Heating the syrup further will indeed give you a little extra time to mix in other ingredients but it will also make the fudge set up harder. Too hot and it won't be fudgy anymore.
Really, without a target temperature you can't tell just by looking at the syrup if it will be the correct texture. It will take some experimenting to figure out how long to boil it.
You might try reducing the quantity of additives, adding the marshmallows before other additives, and stirring faster.
Personally, I wouldn't bother with any fudge recipe that gives times instead of temperatures. There are just too many variables involved and it will take some experimenting and luck to get the texture right.
*Ignoring the creation of crystals for brevity