As it stands, the instructions state to choose Chrome or Firefox from the Source list on the Import screen. As you can see, that option doesn't exist (not for me anyway).
After spending an hour in the live chat support, during which the advisor insulted me on numerous occasions, I found a solution. It's something I didn't want to do and it sort of makes NPM redundant, but I thought I'd share regardless.
I opened a free Lastpass account. I exported my passwords from Chrome as a csv file, imported them into Lastpass, exported them from Lastpass, then pasted them into NPM.
It worked but I'm wondering why Norton makes a simple process extremely difficult?