I'm sitting in a room full of these PAX cabinets that I installed a couple of years ago. Using the leveling feet (as suggested in other answers, and I even agreed in comments), is normally the way you'd want to do this.
However...
There are only holes in the front of the cabinet floor to install the leveling feet, which means that if you screw them down, you're tipping the cabinet even further back.
This means that if the front of the cabinet is not vertical, you'll actually need to drive shims under the back corners of the base to raise the back edge. This will push the cabinet upright (moving the back away from the wall) and leave a gap between the back of the cabinet and the wall.
Once you've got the cabinet vertical (a slight tip back toward the wall is fine - it'll help keep round things from rolling off the shelves), then you'll want to secure the top to the wall using the provided anchors. However, if you just tighten the clamps down, you'll once again pull the top of the cabinet against the wall, tipping it backwards, which isn't what you want.
In order to fasten it to the wall and not tip it, you'll need to use shims to fill the gap at the top so you can securely fasten the anti-tip clamps. If the gap is too large for a pair of shims to fill, you'll want to use a piece of thin wood to fill in some of the space. This piece of wood will be roughly the same thickness as the trim at the bottom of the wall, though a bit thinner is better.
You'll want to take a pair of shims and place one against the wall, thick end down so that it covers the area where the screw goes for the anti-tip clamp. Take another shim, thin end down, place it on top of the first shim and slide it down until it just fills the gap and the cabinet can't rock. Hold the shims in place, then drive the screw through the bracket and into a stud to hold the whole thing in place (note - you have to hold the shims - they'll want to spin when the screw hits them). Do the same in the other top corner and you're done.