These might be stupid questions, but sometimes the simple things are the problem:

1. I am not overly familiar with the "diagnosis" feature you mention, but a simple question i have do you have valid IP address, for both the PG (computer) and the PLC (not a Windows Automatic Private IP that it likes to assign)? I read that you say you can see the IP address for the PLC (how did that get there? from your MPI download?)

2. Can you ping the ethernet address of the PLC?

3. When you assigned the IP to the PLC, did you assign it to a network [i.e. ethenet(1) ]? [ i guess the same would apply to the MIP/DP network.]

4. Do you have ethernet devices that you can "drive down" to via MPI (proving your MPI is linked to the ethernet? (like an HMI)

Just ideas, i am sure someone will have better ideas.