I had the same problem with Yamaha Majesty 125. The seat latch is supposed to be unlocked by turning the ignition key to the left but this did not work. Until I tried it with the key partially pulled out (very little, less than 1 mm). Then there is feeling of some resistance (the cable inside is pulled) and the latch is released, the set can be lifted.
So, try to turn the ignition key to the left when it is inserted not to the very end.
Sergei, April 2009