Option for custom block model no longer available

Hi, I only started using Mcreator this week. From what I've been able to do I find it really useful, but I have 1 problem. I wanted to import a custom model from Blockbench as the block I want to make is not a 16x16px uniform block, but in the drop-down menu for block model when you're creating a new block, the custom option isn't there! When you click the little ? box, "Custom" is still there as an option, but it's not selectable in the drop-down menu.