I was able to load the model and was able to generate the subprocesses (although, I still can't access the diagrams when I try to click the html link)
The fact that you don't have the diagrams is weird, as everything seems to have worked at ths stage. Is there any warning message printed to the screen related to the diagrams? If not, maybe you can check manually where they are.
To this aim, please go to the test_signal/SubProcesses folder (I don’t know if test_signal is the name you choose; if not, please adapt it). There, you should see a few folders starting with P1_ and P2_. There are jpg files in these folders. They include the diagrams.
Does it work?
I launched MG5_aMC and modified the card.dat files:
There is an apparent mistake with the run_card. The PDF-related lines should be lhapdf = pdlabel and 262000 = lhaid (check my version of the run card in the post). This is what causes the compilation failure of MG5aMC.
Un-relately, you have an extra non zero parameter in the paramcard (in the mixing parameters). Also, please don’t forget to set the calculation of the decay width of the heavy neutrino to automatic (auto). I don't know whether you have done it.
Good luck, and please let me know if the comments above help you to fix all issues!
Cheers!