Dishwasher tilting, how do I secure it properly?

[Tigercubrider]

You’d have to pull it out to see, (by lowering the legs)
But they may have just shimmed the front, while it needs one at the back.

As said, it should be side screwed into the adjoining units, but if you pack the top and then adjust the feet to jack it up against the top it should stay

 

[robinbanks]

Genuinely, I'd just carefully drill a hole through the side of the machine, on the world side of the rubber seal

 

[tinpanalley]

Genuinely, I'd just carefully drill a hole through the side of the machine, on the world side of the rubber seal

On the world side, you mean, the side closest to the outside?