How do I get the lid off my Pro Action hand-held steam cleaner?

I have a grey hand-held Pro Action steam cleaner that I haven't used for a few years and I've lost the instructions. I can't open the lid to pour the water inside. The reservoir is an integral part of the device and has one of those circular domed furled lids that I can turn round and round without it coming out. It can be pressed in -- it's a bit like a childproof medicine bottle cap -- but no matter how much I press, pull or turn I can't shift it. I'm going to have to throw it out just because I can't open it!