I am currently travelling on one (in Perugia right now) and have given up on trying to put my itinerary on their site - takes more than an hour of form filling before you realize it's never going to do what you want. Fortunately you can do almost everything without using their site.

It doesn't even seem to work for railway staff, when I did a supplementary reservation at Lecce yesterday the clerk only managed it by using the desktop machine and his own phone simultaneously while the process went so slowly at Interrail's end that a queue of 20 people built up behind me. He looked monumentally pissed off and I could well understand why.