I'm also breaking in my first Inzer belt right now. Same issue! I couldn't get the tongue into the keeper at first, was thinking I should have bought a lever. The first few times I put it on, I was getting TRAPPED in it. Really. Had to wrestle my way out of the thing.
I found a youtube video. I won't link to it here because I think the person sells supplements, but if you search for new lifting belt + how to break in inzer belt you'll see it.
I used his method. Folding it over on itself ever few inches and pressing down on it. And rolling it up back and forth both directions. Just a couple of days of working with it a bit and I can now buckle it and get in and out of it now problem. Also, like T3hPwnisher mentioned, just wearing it around the house doing chores. And wearing it when just doing light squats or whatever at the gym just to break it in.
It's softening up already and I think I'm going to really like it now.