Try this:

sudo apt-get purge ntop; sudo apt-get install ntop

Then the follow screen popps:

**Package configuration

┌─────────────────────────┤ Configuring ntop ├─────────────────────────┐

│ Please enter a comma-separated list of interfaces that ntop should │

│ listen on. │

│ │

│ Interfaces for ntop to listen on: │

│ │

│ none________________________________________________________________ │

│ │

│ │

│ │

└───────────────────────────────────────────────────────────────────

You can get the interface name if you click on the Network Manager in your top panel and select Connection Information. Near the top, you'll see something like "Interface: Ethernet (eth0)". If that's the connection you want to monitor, you'll enter "eth0". (Answer copied and pasted from answered Nov 1 at 19:41Jo-Erlend Schinstad)

I hope I was able to help you.
Cheers.