If you've enabled access through your firewall already (and at last notice, Linksys routers came default with traceroute enabled), then it's most likely your ISP that's blocking.

The only choice is really to contact your ISP and find out what's going on - they should be able to reproduce the results you're seeing. This is usually a pretty easy call to support - although you want to ask about traceroute / TRACERT access rather than Ping Plotter - as everyone is familiar with traceroute/TRACERT - and only a few ISP actively use Ping Plotter.

Posting back here with your results would be most welcome!