Good morning!
Its hard to determine what might be happening from your description. Couple of things you might check, though. First, see if the column that contains your expression values actually got imported as a numeric column. Sometimes there are non-numeric entries that confuse the type-guesser in the importer. If that's the case, delete the column and re-import it, but make sure that you force it to be numeric (click on the column header in the importer). Second, make sure that you are changing the right visual style when you do the mapping -- you should clear "Fill Color" in the STRING Style and then set up your mapping. I'm assuming you got the network from the StringApp in all of this.
-- scooter