A small number of ACs have panel openings that are diagonal, as shown in the example below.
To install the mistbars on this style of AC, you can use a zip tie method shown in this article: http://help.mistbox.com/installation/mistbars/how-to-install-mistbars-when-you-cant-clip-them-into-the-ac-panels
If you're going through the instructions at app.mistbox.com, you will be prompted to select your AC style. On the Select type of panel openings screen, choose Horizontal. When you reach the mistbars step of the instructions, you'll see a video tailored to the installation method you should use.