Figure 14 (Page 12) of the PCA9544 datasheet shows the wiring. You would power the PCA9544 with 3.3V and the pull ups on the Mega will keep that side of the I2C bus at 5V. The MPU-9150 buses would need 3.3V pull ups and everything should be kosher. I'd appreciate if CrossRoads or Grumpy would chime in on that though.
Regarding your earlier question about a 3.3V supply, just get a 3.3V 1117. In addition to the regulator itself you'll just need a 2.2uF to 10uF electrolytic capacitor between the input and GND and a second between the output and GND, keeping those caps as close as possible to the regulator. These SOT-223 packages claim up to 1A of output which is way more than you'd be using here.