I looked up *my* favorite tracking site (not SP) and they don't have it as a choice, either. I checked their support forums (cos I couldn't believe they didn't have the OPTION!). It said they don't do it because most LABELS don't track it!
You can build your own items on myfitnesspal (like sweet potato, and then fill out ALL the nutrients). That's a lot of work unless you only eat a small 'variety' of foods. Personally, I eat a small variety (especially since I eat only at home and rarely processed from supermarkets) but maybe others don't.
Sorry! I spent about 10 minutes looking for you!
I'll also recommend nutritiondata.com (they have exceptional data for food but it's a LOT of info!)