Maybe this will help
These axes only relate to the chip. In the picture the y axis is up, but if you were to lay the chip in the picture down z axis would be up. You can mount the chip however you want to get the acceleration in the axis you want. I'm not 100% sure, but I feel like if you were using the accelerometer as a tilt sensor then you might run into some problems.
Not sure if you know this, but most accelerometers like this one
have notations on the PCB to show you the +- direction of the axis. In the picture it's pretty easy to see x/y +-, but z+ is into the page. The way I know this is by imagining an arrow with a tail on the back, then imagining what it would look like from the front or the back of the arrow. When looking at the back of the arrow it's draws with the tail(feathers) on it, but when draws from the front it is drawn like a dot, showing just the head of the arrow.
Hope this helps.