These Sun type 411 chassis can be a little bit tricky to open.
There is no need to break the somewhat brittle plastic latches, simply follow the guide on http://eintr.net/systems/sun/sun411/ and you will be fine.
Insert screwdriver from the 3rd hole from the back on each side.
I made a copy of the two most important images and uploaded here. Copyright goes to Hugo Villeneuves.