My guess too is that your PSP is showing the flash instead of the memory stick. So it's either you put it back to memory stick by entering VSH Menu or Recovery Menu (or even using XMB Control). But if you don't know those. The other safe way to do is hook you memory stick into a card adaptor. This way, it won't read any flash thingy because it's not connected to a PSP, thus allowing you to see the ISO and PSP folders.