I have the same problem. I opened mine with a razor blade. You just have to cut around the plug and then along the plug and pull the rubber off. Unfortunately, the wire is frayed very close to the plug and I was uncomfortable working that close. In the end I broke the plug and now am probably going to have to buy a new supply for $70. I can't believe I have to spend that much to replace a simple plug.
My AC power adapter broke. The wire inside the cord just as it enters the plug is broken/frayed. (I can see it spark). How can I open the plug and repair this. If I could figure out how to open it, I could cut back on the wire and re-solder it if necessary. Any advice would be much appreciated.