can't find nintendo ds emulator for psp..

It's called something like DSemu.

Though I'm sure it works like crap, I tried it sometime last year and it was completely unplayable. I know progress can be made, but I don't think it is possible.

It runs at less than 10 FPS last I checked. Not even worth it. If you must emulate the DS, just use the various ones for the PC.

P.S. Shift key. Learn it. Live it. Love it.