the data stream mod, when in place works like this....
the 8 toggle switches you've put in which connect the data streams, when all of them are on, do nothing. the keyboard should be the same as normal...
now, when you turn one or a few (i wouldn't do all of them at once) of the switches to the off position and then change patch, press a key or two and then turn them all on again. you should hear a patch which has parts of the former selected sound in it. chances are it will be a bit more random than you expect. eg, you can't think, hmmm, i'll mix a piano patch with a string patch. well, you can, but the bends don't seem to be the same every time.
it's also worth noting that whilst any of the data streams are off, the keyboard can exhibit 'crash' type tendencies where the keyboard won't track pitch or make a sound at all, sounds will only play when three or four keys are pressed etc. the drums get a bit, er, funky too.
so, if you've wired up the switches, turned them all on and having issues.... then i'd suggest you've done something a bit odd. maybe soldered something wrong or not made a complete circuit on one of the data streams. my pss-270 didn't work correctly when i first wired it up but gentle tugging on the soldered points soon exposed a dodgy connection. it's all a bit fiddly...