As far as I know, you can only edit the information on foods that you have entered manually yourself. If there is no existing entry that matches what you are eating, then you'll have to enter it yourself.
For supplements, you can check to see if there is an existing entry for the ones you use. If not, then again you would enter it yourself. You treat them just like another food.
One thing to keep in mind is that potassium, magnesium, and folate are not nutrients that are required to be listed on food labels, so it is possible (probable) that you are actually getting more of these than the tracker may indicate. You might want to check out a site like nutritiondata.self.com for the complete nutrient list for the packaged foods that you eat so that you can include more complete data for your manually entered items.
It is time-consuming at first to get all of your regular foods entered completely, but once they're in the system and in your "favourites" then they take no additional time and you know that you are getting the most accurate information possible.