You can't sim 3D shapes (such as wedge or airfoil shapes in the fins, so the sides will appear flat.
You can, however, capture the 2D profile of the fin shape. You must convert the profile of the fin into an image such as a .png, .jpg, or .bmp format. The fin will be a solid black, the surrounding area needs to be white. The fin must point as if the left side of the image is the forward edge, and the root edge is even with the bottom of image. Negative X values are not permitted... So, if you want to have a detail such as the controlling vanes of the Mercury Redstone present, you'll need to attach the fin to a phantom body tube, OR use a fin tab to simulate it.
Then in OpenRocket, you'll need to add a freeform fin, click on fin shape tab, and import from image.
After importing, the image will likely be MASSIVELY overscale. So, you'll need to scale the fin after importing it. After scaling you'll need to check the thickness of the fin (it gets scaled too).
Hollows in the fin will need to be ignored, or you'll need to "cut" the fin to allow the software to follow the edges. WARNING!!! On fins with lots of curves, the hollows are EXTREMELY difficult to represent, and the fin may need to be cut into several segments and shifted around to get a representation of that shape. I know... I've tried.
I got it solved . It was a pain because openrocket angle 0 ref was different from the 0 angle ref in the cad program I was using
Gotta get some fins laser cut to keep a uniform fin pattern as when I get my build started gotta make 3 versions of the 4 fin Black Brant