How to match Activities to Contacts during import?
Export the contacts with their contact ID, first name and last name.  (Hopefully you have a way of searching for just the new contacts, but if not, exporting a larger group of contacts will be OK)

Open each CSV in something like Excel or Calc and create a combined "First Last name column in both.  You can then use vlookup (in Excel) or the equivalent in Calc to add the contact id to the list of activites you want to import.