What is a nutrient for yeast?
They're likely referring to di-ammonium phosphate, which is used in wine making and mead brewing, as it provides nitrogen to accelerate the yeast growth.

I found a site giving instructions for using egg whites as a replacement, but it might be easier (and safer) to just go to a shop that specializes in brewing or wine making supplies and getting the right stuff.  (I have no idea how expensive it is, as I've never done it)

Your other option would be to find an alternate recipe that doesn't call for it ... it might take longer to brew, though.